GIFT   OF 

MICHAEL  REESE 


THE 


DETERMINATION 


OF 


ROCK-FORMING  MINERALS. 

BY 

DR.    EUGEN    HUSSAK, 


PRIVAT-DOCENT  IN  THE  UNIVERSITY  OF  GRAZ. 


WITH  ONE  HUNDRED  AND  THREE  WOODCUTS. 


AUTHORIZED  TRANSLATION  FROM  THE  FIRST  GERMAN  EDITION 

\ 

BY 

ERASTUS  G.  SMITH,  PH.D., 

PROFESSOR   OF   CHEMISTRY  AND   MINERALOGY,   BELOIT  COLLEGE, 
BELOIT,  WISCONSIN. 


NEW  YORK  : 

JOHN     WILEY    &    SONS. 

1886. 


Copyright,  1885,  by 

3  ^3  03 
JOHN  WILEY  &  SONS. 


\    \ 


EARTH 

SCIENCES 

LIBRARY 


PREFACE  TO  THE  FIRST  ENGLISH 
TRANSLATION. 


THE  following  authorized  translation  of  Dr.  Hussak's  work 
was  undertaken  with  the  view  of  supplying  a  want  felt  in  our 
colleges  and  universities.  Though  great  progress  has  been 
made  in  the  sciences  of  mineralogy  and  lithology  in  later 
years,  through  the  study  of  the  optical  and  the  other  physical 
properties  of  minerals,  few  attempts  have  been  made  to  con- 
dense the  exhaustive  and  original  articles  scattered  through 
the  scientific  periodicals  and  State  and  national  publications, 
and  to  put  them  in  suitable  shape  for  use  in  the  laboratory 
and  the  class-room.  It  has  been  the  aim,  therefore,  to  place 
before  the  American  student  a  practical  work  which  shall  de- 
scribe the  methods  and  exhibit  the  results  of  such  investiga- 
tions. 

The  translation  of  a  technical  work  of  this  character  is  beset 
with  difficulties  appreciable  only  by  those  who  have  under- 
taken a  similar  task.  Few  liberties  have  been  taken  with  the 
text,  and  the  attempt  has  been  made  to  reproduce  the  origi- 
nal literally  so  far  as  possible  ;  at  points,  however,  in  order  to 
convey  clearly  the  author's  meaning,  some  recasting  of  sen- 
tences was  unavoidable. 


IV         PREFACE    TO    THE  FIRST  ENGLISH   TRANSLATION. 

The  translator  will  consider  it  a  great  favor  if  any  errors 
noticed  either  in  statement  or  in  translation  are  communi- 
cated to  him,  in  order  that  they  may  be  eliminated  in  future 
editions. 

I  would  express  my  thanks  to  Dr.  George  Williams  of  Balti- 
more for  several  such  corrections  to  the  original  text  already 
received  and  incorporated  in  the  translation. 

ERASTUS  G.  SMITH. 

BELOIT  COLLEGE,  BELOIT,  WISCONSIN,  October,  1885. 


PREFACE. 


As  the  following  Manual  is  designed  especially  for  the  use 
of  students,  the  cost  of  the  work  demanded  as  much  abridg- 
ment as  possible.  For  this  reason  much  of  the  knowledge  of 
minerals  which  belongs  to  mineralogy  proper  is  passed  over,  and 
in  the  bibliography  of  Part  II.  only  those  works  are  cited  which 
contain  detailed  communications  concerning  the  microscopical 
properties  of  the  rock-forming  minerals. 

I  must,  at  this  time,  express  my  gratitude  to  Professor  Dr. 
F.  Zirkel  for  his  many  friendly  suggestions ;  nor  am  I  under 
less  obligations  to  Professor  F.  Fouqu£,  who  has  most  cour- 
teously allowed  the  reproduction  of  a  large  number  of  figures 
from  his  well-known  work,  "  Min£ralogie  Micrographique." 

EUGEN    HUSSAK. 

GRAZ,  November,  1884. 


TABLE   OF   CONTENTS. 


PART    I. 
METHODS  OF   INVESTIGATION. 

PAGE 

Preparation  of  Microscopical  Sections 3 

The  Microscope  provided  with  Polarization  Apparatus  suitable  for  Mine- 

ralogical  and  Petrographical  Investigation 7 

A.  Optical  Methods  of  Investigation 16 

1.  Examination    of    Mineral    Cross-sections    in    Parallel    Polarized 

Light 16 

I.  Single  refracting  Minerals 17 

II.  Double-refracting  Minerals, 17 

2.  Examination  of  Minerals  in  Convergent  Polarized  Light 29 

3.  Behavior  of  Twinned  Crystals  in  Polarized  Light 37 

Twins  of  the  Regular  System 37 

Twins  of  the  Tetragonal  and  Hexagonal  Systems 38 

Twins  of  the  Rhombic  System 39 

Twins  of  the  Monoclinic  System 40 

Twins  of  the  Triclinic  System 43 

4.  Determination  of  the  Index  of  Refraction 44 

5.  Pleochroism  of  Double-refracting  Minerals 45 

Determination  of  the  Axial  Colors 45 

B.  Chemical  Methods  of  Investigation 50 

Microchemical  Methods 51 

a,  Boficky's  Microchemical  Method 55 

b.  Behrens's  Microchemical  Method 59 

C.  Mechanical  Separation  of  the  Rock-forming  Minerals 66 

I.  Separation  with  the  Solution  of  the  Iodides  of  Potassium  and 

Mercury 67 

II.     Klein's  Solution 73 

III.  Rohrbach's  Solution  of  the  Iodides  of  Barium  and  Mercury 75 

IV.  Methods  of  Separation  based  on  the  Different  Action  of  Acids 

on  Minerals ?6 

V.   Separation  of  the  Rock-constituents  by  means  of  the  Electro- 
magnet   79 


VI 11  TABLE   OF  CONTENTS. 


D.  Explanations  of  the  Tables  relating  to  the  Morphological  Properties  of 

the  Rock  forming  Minerals 81 

I.   Mode  of  Occurrence  of  the  Rock-constituents 81 

II.   Structure  of  the  Rock-forming  Minerals 87 

Shell-formed  Structure  of  Crystals go 

Interpenetration  of  the  Rock-constituents 93 

III.   Inclosures  of  the  Rock-forming  Minerals 93 

Gas  pores 94 

Fluid  Inclosures 95 

Inclosures  of  Vitreous  Particles 97 

Inclosures  of  Foreign  Minerals 99 

IV.  Decomposition  of  the  Rock-constituents 101 

PART    II. 
TABLES   FOR   DETERMINING   MINERALS. 

Table  for  Determining  the  System  of  Crystallization  of  the  Rock-forming 
Minerals 106 

A.  Even  in  the  thinnest  Sections  of  Opaque  Minerals 108 

B.  Minerals  Transparent  in  Thin  Sections 112 

I.  Single  'refracting  Minerals. 112 

a.  Amorphous  Minerals 112 

b.  Minerals  Crystallizing  in  the  Regular  System 114 

II.   Double-refracting  Minerals 122 

a.  Optically  Uniaxial  Minerals 122 

1.  Minerals  Crystallizing  in  the  Tetragonal  System 122 

a.   Double-refraction  Positive 122 

ft.   Double-refraction  Negative 124 

2.  Minerals  Crystallizing  in  the  Hexagonal  System 128 

a.    Double-refraction  Positive 128 

ft.   Double-refraction  Negative 132 

b.  Optically  Biaxial  Minerals 140 

1.  Minerals  Crystallizing  in  the  Rhombic  System 140 

2.  Minerals  Crystalling  in  the  Monoclinic  System 156 

3.  Minerals  Crystallizing  in  the  Triclinic  System 178 

C.  Aggregates 189 

Bibliography   197 

Explanation  of  Cuts  accompanying  Part  II 215 

Cuts  accompanying  Part  II 219 

Index..  229 


ON  THE   DETERMINATION   OF  ROCK- 
FORMING   MINERALS. 


PART    I. 

METHODS  OF  INVESTIGATION. 


F.  ZIRKEL.     Die  mikroskopische  Beschaffenheit  der  Mineralien  und  Gesteine. 

Leipzig,  1873. 
H.  ROSENBUSCH.    Mikroskopische  Physiographic  der  petrographisch  wichtigsten 

Mineralien.     Stuttgart,  1873. 

FOUQUE  ET  MICHEL  LEVY.     Mineralogie  micrographique.     Paris,  1878. 
E.    COHEN.      Zusammenstellung    petrographischer    Untersuchungsmethoden. 

Strassburg,  1884. 

THERE  are  two  methods  of  examining  rocks,  the  macro- 
scopical  and  the  microscopical. 

In  the  macroscopical  investigation  of  rocks  those  parts  of 
the  mineral  mixture  discernible  with  the  naked  eye  can  be 
studied  with  reference  to  crystalline  form,  cleavage,  color, 
lustre,  streak,  hardness,  solubility  in  acids,  etc.  For  the  more 
exact  optical  investigation,  however,  cleavage  sections  exactly 
oriented  must  be  obtained,  the  cleavage  angle,  when  possible, 
measured  in  order  to  determine  the  plane  of  cleavage,  and  the 


2  DETERMINATION  OF  ROCK-FORMING  MINERALS. 

section,  if  not  already  transparent,  ground  thin.  Such  an 
investigation  of  the  rock-forming  minerals  leads  in  most  cases 
to  the  goal/ provided  the  particles  have  a  certain  magnitude, 
at  least  1-2  mm.  Isolated  particles  of  minerals  can  be 
examined  before  the  blow-pipe  ;  yet,  because  of  their  minute- 
ness, such  a  purely  macroscopical  examination  is  insufficient 
in  most  cases.  This  is  especially  true  in  porphyritic  or  very 
fine-grained  rocks,  and  therefore  for  these  rocks  the  micro- 
scopical examination  is  employed.  It  is  necessary  in  such  a 
case  that  the  pieces  of  rock  under  examination  shall  be  ground 
into  thin  transparent  leaves.  In  such  sections  the  single  con- 
stituents are  cut  in  most  varied  directions.  By  these  minute 
cross-sections  the  crystals  and  the  rock-forming  minerals  can 
be  determined  by  optical  methods  with  the  polarization-micro- 
scope, and  by  combination  of  the  optical  with  the  crystallo- 
graphical  properties,  i.e.,  with  the  form  of  the  cross-section, 
i.e.,  crystalline  form,  and  cleavage. 

This  determination  is  more  difficult  if  the  minerals  occur 
only  as  grains.  Of  course  here  also  the  human  eye  has  its 
limitations:  if  the  separate  particles  are  so  minute  that  they 
cannot  be  observed  in  section,  i.e.,  afford  no  cross-sections ; 
or,  when  examined  under  the  highest  possible  magnifying 
power,  they  give  no  figures  suitable — i.e.,  large  enough — for 
optical  study,  their  determination  by  the  polarization-micro- 
scope is  impossible. 

In  the  following  pages  is  given  a  description  of  the  method 
of  producing  preparations  from  rocks  suitable  for  microscopi- 
cal study ;  of  the  application  of  the  polarization-microscope 
adapted  to  the  complete  exposition  of  the  optical  and  chemical 
methods  of  determination  ;  then  follows  the  discussion  of  the 
mechanical  separation  of  the  rock-constituents  according  to 
their  specific  gravity  and  by  the  electro-magnet ;  and,  finally, 
a  short  chapter  on  the  structure  of  the  rock-forming  minerals 
and  a  systematic  survey  of  them. 


METHODS  OF  INVESTIGATION.  VV  C'          3 


The  Preparation  of  Microscopical  Sections. 

In  order  to  prepare  a  thin  section  from  a  rock,  either  a 
suitable  tablet  is  cut  with  a  section-cutter  from  the  rock,  or  a 
convenient  fragment,  about  2  ccm.  large,  is  broken  off  with  a 
hammer,  and  as  even  a  face  as  possible  is  ground,  using  either 
an  emery-disk  of  a  section-grinder  or  grinding  by  hand  on  an 
iron  plate  with  coarse  emery-powder  and  water.  The  size  of  the 
emery  used  depends  entirely  upon  the  hardness  of  the  rock. 
Evenness  of  the  emery-powder,  and  an  iron  plate  as  smooth  as 
possible  and  free  from  furrows,  are  chief  factors  in  obtaining 
an  even  surface  on  the  ground  fragment. 

If  the  face  is  sufficiently  even,  it  is  polished  on  a  glass  plate 
with  fine  floated  emery,  or  emery-flour,  and  water.  The  frag- 
ment is  then  cemented  by  this  face  with  boiled  Canada  balsam 
to  an  ordinary  glass  plate  (preferably  one  that  is  quadratic) 
somewhat  larger  than  the  fragment,  and  rather  thick,  so  that 
it  can  be  better  grasped. 

Certain  precautionary  measures  must  be  observed.  The 
fragment  must  be  first  well  cleaned  and  dried,  the  Canada  bal- 
sam sufficiently  heated,  boiled  neither  too  much  nor  too  little, 
so  that  the  emery-powder  may  not  become  distributed  through 
it  or  the  balsam  crack  off  from  the  glass.  The  balsam  may  be 
boiled  over  an  alcohol-lamp,  either  in  an  iron  spoon  or  directly 
on  the  object-glass.  Care  must  be  taken  that  the  balsam  does 
not  inflame.  It  is  impossible  to  state  the  exact  instant  when 
the  balsam  is  sufficiently  boiled,  as  this  depends  on  its  state  of 
dilution  and  must  be  determined  by  several  experiments.  The 
balsam  is  sufficiently  boiled  if,  after  it  has  already  begun  to 
fume  rather  strongly,  large  bubbles  rise  from  the  bottom,  or 
the  balsam  begins  to  evaporate  from  the  edges  of  the  glass 
plate.  The  boiling  of  the  balsam  is  conducted  most  safely  in 


4  DETERMINATION  OF  ROCK-FORMING  MINERALS. 

an  oven  with  thermometer  attachment,  such  as  are  sold  by 
Fuess  of  Berlin.  If  the  balsam  has  been  boiled  in  an  iron 
spoon,  a  small  portion  is  placed  on  an  object-glass,  and  this 
gently  warmed  until  the  balsam  becomes  a  thin  liquid. 

The  evenly-ground  rock-section  is  firmly  pressed  into  the 
boiled,  still  fluid,  balsam,  with  the  plane  surface  downward.  In 
the  operation  care  must  be  taken  that  no  bubbles  of  air  remain 
between  the  rock  and  glass,  as  often  happens  when  the  surface 
ground  on  the  rock  is  not  perfectly  even.  The  plate  thus  pre- 
pared is  allowed  to  thoroughly  cool.  If  the  balsam  on  the  plate 
about  the  rock-tablet  receives  no  impression,  or  appears  free 
from  fissures,  it  is  sufficiently  boiled. 

The  natural  surface  of  the  rock-fragment  is  next  ground 
with  coarse  emery-powder.  This  is  continued  until  the 
larger  mineral  particles  or  even  the  plate  itself  begins  to  be 
translucent,  i.e.,  until  the  thickness  is  about  -J-i  mm.  Here, 
again,  care  must  be  exercised  that  the  surface  is  as  even  as  pos- 
sible, and  that  the  Canada  balsam  surrounding  and  protecting 
the  plate  is  not  completely  cut  away.  The  grinding,  as  before, 
is  continued  on  glass  plates  with  fine  emery,  and  finally  with 
emery  flour,  until  the  tablet  of  rock  becomes  perfectly  trans- 
parent. It  is  then  cleaned  from  the  emery,  the  surrounding 
Canada  balsam  is  carefully  scratched  away,  and  is  then  dried. 
For  the  final  preparation  a  better  object-glass  is  selected,  one 
well  polished  and  freed  from  dust-particles  or  clinging  threads, 
dried,  and  a  larger  drop  of  Canada  balsam  placed  upon  it.  The 
balsam  may  be  boiled  directly  on  the  object-glass  or  in  a  spoon, 
and  then  transferred  as  in  the  previous  case. 

The  thin  rock-tablet,  to  which  another  small  drop  of  balsam 
has  been  added,  is  made  movable  by  carefully  and  gently 
heating  the  object-glass,  and  with  a  pointed  bit  of  wood  is 
pushed  over  on  to  this  second  glass,  which  in  turn  is  gently 
warmed  so  that  the  balsam  again  becomes  mobile  and  sur- 
rounds the  rock-section  on  every  side ;  the  covering-glass,  of 


METHODS  OF  INVESTIGATION.  5 

course  previously  cleaned  and  warmed,  is  laid  upon  and  care- 
fully pressed  down  upon  the  rock-section  so  that  the  excess 
of  balsam  and  the  air-bubbles  escape.  The  preparation  is 
allowed  to  cool  slowly  until  the  balsam  has  solidified,  and  is 
then  cleaned  by  carefully  scratching  away  the  excess  of  balsam 
with  a  knife  and  washing  with  alcohol. 

As  by  scratching  away  the  balsam  the  covering-glass  is 
often  liable  to  break  away,  owing  to  the  overheating  of  the 
balsam,  it  is  advisable  to  shave  away  the  balsam  with  a 
warmed  knife,  and  then  wash  the  preparation  with  alcohol. 

Many  rocks,  especially  those  of  a  coarse  granular  structure, 
exceedingly  porous  or  decomposed,  cannot  thus  be  transferred, 
and  are  shattered  in  the  preparation.  Sections  from  such 
rocks  therefore  must  be  placed  on  a  better  object-glass  at 
once,  and,  after  they  are  ground  thin,  must  be  finished  on  this 
same  glass  by  pouring  boiling  balsam  on  the  dried  and  cleaned 
section,  and  the  rapid  laying  on  and  gentle  pressing  down  of 
the  covering-glass.  Here  care  must  be  taken  neither  to  warm 
the  object-glass  a  second  time,  nor  to  press  down  the  covering- 
glass  too  firmly,  as  in  either  case  the  section  is  often  broken  ; 
it  is  therefore  necessary  to  finish  the  preparation  as  rapidly  as 
possible  in  order  that  the  balsam  upon  the  glass  may  not  cool 
and  thus  necessitate  a  second  warming. 

Such  rocks  as  pumice-stone  which  are  exceedingly  porous 
or  full  of  cavities, 'or  of  a  drossy  character,  or  friable  and 
fragile,  as  tufa,  must  be  boiled  in  Canada  balsam  first,  to  make 
possible  the  grinding  of  a  plane  surface,  as  the  balsam  forcing 
its  way  into  the  cavities,  and  becoming  solid  on  cooling, 
imparts  to  the  whole  a  greater  degree  of  consistency.  Such 
thin  sections  must  of  course  be  finished  according  to  the 
method  last  described,  upon  the  same  object-glass  on  which 
it  was  ground. 

Sections  easily  shattered  may  be  prepared  most  safely  by 
Canada  balsam  dissolved  in  ether  or  chloroform.  The  prepara- 


0  DETERMINATION  OF  ROCK-FORMING  MINERALS. 

tion  must  not  be  heated,  and  must  be  allowed  to  dry  very 
slowly.  It  is  advisable  to  use  rather  more  balsam  than  is 
ordinarily  taken,  as  in  the  process  of  drying,  i.e.,  the  evapora- 
tion of  the  ether,  air-bubbles  may  enter  the  balsam  beneath  the 
covering-glass.  It  is  also  advisable  to  avoid  spotting  the 
covering-glass  with  balsam,  as  cleaning  the  preparation  cannot 
be  undertaken  for  several  weeks,  until  after  the  balsam  is  com- 
pletely dried. 

Thoulet  has  described  a  method  of  cutting  isolated  mineral 
particles,  sand,  etc. 

The  powder  to  be  examined  is  mixed  with  about  ten  times 
its  volume  of  zinc  oxide,  and  the  mixture  is  rubbed  up  to  a 
thick  mud  with  potassium  silicate  (soluble  glass).  This  is  then 
pressed  into  a  mould  conveniently  made  from  a  short  piece  of 
thick  glass  tubing,  placed  on  an  object-glass,  and  allowed  to 
stand  several  days  and  harden.  When  thoroughly  dried,  the 
mass  is  easily  slipped  from  the  glass,  is  solid,  and  can  be  worked 
into  a  thin  section  exactly  as  any  rock  fragment. 

In  order  to  grind  friable  rocks,  or  those  become  rotten 
through  advanced  decomposition,  according  to  A.  Wichmann 
(Tschermak's  Min.  u.  petr.  Mitth.  V,  1882,  33)  the  best  course 
is  the  following :  The  fragment  broken  away  is  first  shaved  on 
one  side  as  even  as  possible  with  a  knife,  and  this  is  polished 
on  a  dry  glass  plate ;  the  fragment  is  then  cemented  to  the 
plate  with  Canada  balsam,  previously  cooled  so  that  the  rock 
may  not  be  further  changed  by  its  high  temperature,  and 
again  shaved  on  the  opposite  side  until  as  thin  a  section  as 
possible  remains,  which  is  finally  prepared  with  Canada  balsam 
dissolved  in  ether. 

The  Material  for  the  Preparations. 

The  emery  should  be  as  pure  as  possible,  i.e.,  unadulterated  and  rich  in 
corundum,  the  size  of  the  coarser  granules  about  0.3-0.5  mm.;  the  fine  emery 
should  be  like  flour.  The  coarser  variety  is  known  as  "  No.  70,"  the  fine  variety 
as  "emery  flour." 


METHODS  OF  INVESTIGATION.  7 

The  Canada  balsam  should  be  clear  and  rather  liquid. 

The  object-glasses  are  not  generally  more  than  18  mm.  square. 

Labels  for  microscopical  preparations  are  to  be  had  in  book-form. 

Thin  rock-sections  are  prepared  by  Fuess,  Berlin,  S.  W.,  Alte  Jakobstrasse, 
108,  and  by  Voigt  &  Hochgesang,  Gottingen;  large  collections,  also,  of  thin 
sections,  systematically  arranged,  can  be  obtained  from  the  same  firms.  Both 
houses  supply  excellent  microscopes  especially  adapted  to  mineralogico- 
petrographical  investigations. 


The  Microscope  provided  with  Polarization  Apparatus 
suitable  for  Mineralogical  and  Petrographical  In- 
vestigation. (Also  often  called  the  "  polarization-micro- 
scope.") 

TIL  LIEBISCH.  Bericht  iiber  die  wissenschaftlichen  Instrumente  auf  der  Ber- 
liner Gevverbeausstellung.  Berlin,  1879,  P-  342- 

H.  ROSENBUSCH.     N.  Jahrb.  f.  Miner,  u.  Geol.     1876,  p.  504. 

Ueber  die  Anwendung  der  Condensorlinse  bei  Untersuchungen  im  con- 

vergentpolarisirten  Lichte: 

v.  LASAULX.     N.  Jahrb.  f.  Miner,  u.  Geol.     1878,  p.  377. 

E.  BERTRAND.  Societ6  mineralogique  de  France.  1878,  9  Mai  p.  22  and 
14  Nov.  p.  96. 

C.  KLEIN.     Nachr.  d.  k.  Ges.  d.  Wissensch.  z.  Gottingen.     1878,  p.  461. 
Ueber  stauroskopische  Methoden: 

H.  LASPEYRES.     Groth's  Zeitschr.  f.  Krystallographie,  VI.  Bd.  p.  429. 

L.  CALDERON.     Groth's  Zeitschr.  f.  Krystallographie,  II.  Bd.  p.  68. 

The  completely  equipped  polarization-microscope  (Figs. 
I  and  2)  differs  from  the  ordinary  microscope  by  (i)  the 
presence  of  a  graduated  object-stage  revolving  horizontally 
(Fig.  I,  c),  with  vernier  attachment  suitable  for  the  determina- 
tion of  the  directions  of  extinction,  measurement  of  angles, 
etc.;  (2)  two  Nicol's  prisms  (Fig.  2,  ss  and  rr)  for  investiga- 
tions in  parallel  polarized  light ;  (3)  a  condenser  (Fig.  2,  TT) 
for  investigations  in  converging  polarized  light ;  (4)  a  plate  of 
quartz  (Fig.  2,  ZZ)  for  determining  feebly  double-refracting 
minerals,  which  is  cut  perpendicular  to  the  chief  axis,  has 
parallel  planes,  and  can  be  introduced  over  the  objective  by 


8  DETERMINATION  OF  ROCK-FORMING  MINERALS. 

a  slit  (Fig.  2,  ti) ;  (5)  a  calcite  plate  for  stauroscopic  investi- 
gations cut  perpendicular  to  the  chief  axis,  with  parallel  planes 
—that  is,  a  Calderon's  double-plate  (Fig.  2,  c)  or  a  Brezina's 
calcite  plate  set  in  an  ocular;  (6)  a  fourth  undulation  mica 
plate  and  a  Dove's  quartz  compensation-plate,  i.e.,  a  thin 
wedge  of  quartz  for  the  determination  of  the  character  of  the 
double -refraction,  which  either  enters  or  is  just  below  the 
analyzer;  and  finally  (7)  an  apparatus  for  centring  the  ob- 
ject-stage (Fig.  I,  m  and  ;/ ;  Fig.  2,  N,  mi,  mm),  and  various 
minor  pieces  of  apparatus,  as  the  cross-threads  in  the  ocular, 
an  ocular-  and  stage-micrometer,  blende  (Fig.  2,  dd)  for  in- 
vestigations in  converging  polarized  light,  the  graduation  of 
the  head  of  the  micrometer-screw  and  of  the  plate  on  the  stage. 

For  mineralogico-optical  investigations  one  Nicol's  prism, 
the  polarizer  (Fig.  2,  rr\  is  fixedly  adjusted  beneath  the 
stage  and  above  the  reflector ;  and  the  second,  the  analyzer 
(Fig.  2,  ss),  is  graduated  and  is  above  the  ocular.  For  in- 
vestigations in  parallel  polarized  light  it  is  very  convenient  if 
the  polarizer  is  fixed  in  such  a  position  that  the  directions  of 
vibration  of  both  nicols  are  at  right  angles  to  each  other, 
i.e.,  the  nicols  are  crossed  when  the  zero-point  of  the  analyzer 
coincides  with  a  mark  on  the  tube,  and  at  the  same  time  the 
ocular  with  its  cross-threads  so  adjusted  in  the  tube  that  the 
arms  of  the  cross-threads  are  exactly  parallel  with  the  direc- 
tions of  vibration  of  both  nicols. 

If  this  is  not  the  case,  the  nicols  must  always  first  be  crossed 
by  turning  the  analyzer  until  complete  darkness  occurs  and 
this  position  of  the  analyzer  is  noted.  Moreover,  the  arms  of 
the  cross-threads  must  be  parallel  to  the  nicol  chief  sections. 
This  may  be  done  in  the  following  manner:  We  place  on  the 
stage  of  the  microscope,  and  between  the  crossed  nicols,  an 
object-slide  to  which  is  firmly  cemented  either  a  small  quartz- 
crystal  or  a  rock-section  containing  a  longitudinal  section  of  an 
apatite  crystal,  and  turn  the  stage  until  the  quartz  or  the 


METHODS  OF  INVESTIGATION. 


FIG.  i.— POLARIZATION-MICROSCOPE,  BY  R.  FUESS.    (New  model.) 


IO         DETERMINATION  OF  ROCK-FORMING  MINERALS. 

apatite  crystal  is  completely  darkened.  The  analyzer  is  now 
removed-  from  the  ocular,  and  the  ocular  is  revolved  until  one 
arm  of  the  cross-threads  within  the  ocular  is  exactly  parallel  to 
the  prismatic  edge  of  the  quartz  crystal  or  the  longitudinal 
edge  of  the  apatite  needle.  In  order  to  determine  the  direc- 
tions of  extinction  in  minerals,  care  must  be  taken  that  the 
ocular  carrying  the  cross  threads,  when  correctly  placed  in  the 
manner  described,  is  not  displaced,  as  can  easily  occur  in  re- 
moving the  analyzer. 

The  Condenser  (the  Lasaulx-Bertrand  lens)  for  producing 
converging  polarized  light  in  the  microscope  is  formed  from 
two  plano-convex  lenses.  One  of  these  is  screwed  directly 
above  the  polarizer,  and  the  second,  in  a  suitable  setting,  laid 
upon  the  first  (Fig.  2,  TT).  In  investigations  in  convergent 
light,  the  ocular  is  removed  and  the  nicols  crossed.  Objec- 
tive 7  and  ocular  3,  Hartnack,  is  the  best  combination,  though 
a  more  acute  objective  system  can  often  be  advantageously 
employed.  In  examining  very  diminutive  crystalline  cross- 
sections,  a  blende  (Fig.  2,  dd)  is  placed  above  the  analyzer  for 
the  purpose  of  isolating  the  cross-section  to  be  examined.  The 
Bertrand  lens  can  be  inserted  within  the  tube  in  place  of  the 
ocular  (removed  for  the  purpose),  should  an  enlargement  of  the 
interference-figures  be  required. 

The  Biot-Klein's  Quartz  Plate  (Fig.  2,  ZZ),  about  2  mm.  thick, 
with  parallel  planes  cut  perpendicular  to  the  optic  axis,  and 
brass-mounted,  is  introduced  through  a  suitable  opening 
directly  above  the  objective  (Fig.  2,  tt).  In  order  to  use  this 
quartz  plate  in  examining  feebly-refracting  minerals  or  those 
of  marked  zonal  structure,  the  upper  nicol  is  revolved,  after 
the  quartz  plate  is  introduced  and  the  polarizer,  objective,  and 
ocular  are  in  suitable  positions,  until  the  extremely  sensitive 
red  (the  so-called  "  teinte-sensible")  of  the  circular  polarizing 
quartz  appears.  The  mineral  to  be  examined  is  then  placed 
beneath  the  objective. 


METHODS  OF  INVESTIGATION. 


FIG.  2.— POLARIZATION-MICROSCOPE,  BY  R.  FUESS.    (Older  model.    Cross-section.) 


12          DETERMINATION  OF  ROCK-FORMING  MINERALS. 

Minerals  with  feeble  double-refraction,  as  leucite,  or  those 
showing  optic  anomalies,  as  garnet,  will  induce  a  change  of 
color. 

The  quartz  plate  is  also  applied  to  the  more  exact  determi- 
nation of  the  position  of  the  directions  of  vibration,  as  ail 
double-refracting  minerals  undergo  a  change  of  color,  and  this 
remains  unchanged  only  in  isotropic  sections  or  when  an  axis 
of  elasticity  coincides  with  a  nicol  chief  section. 

The  Calcite  Plate,  about  2  mm.  thick,  with  parallel  planes, 
and  cut  perpendicular  to  the  optic  axis,  is  set  in  a  cork  ring, 
and  when  in  use  is  laid  between  the  ocular  and  the  analyzer. 
The  nicols  are  crossed,  and  the  interference-figures  of  the 
calcite  plate  then  appear  on  the  section  under  examination. 
The  arms  of  the  cross-threads  must  again  coincide  with  the 
arms  of  the  interference-cross  of  the  calcite  plate.  More  exact 
stauroscopic  investigations  cannot  be  undertaken  with  this 
plate  except  on  the  larger  mineral  sections. 

For  the  microstauroscopical  measurements  the  Calderon 
Double-plate  (Fig.  2,  c]  is  peculiarly  adapted.  This  is  made 
from  a  twin  of  calcite  artificially  formed  (Fig.  3,  abcdef)  by 
cutting  a  rhombohedron  through  the  short 
diagonals,  grinding  away  a  wedge-shaped  por- 
tion from  either  half,  and  again  cementing  the 
polished  surfaces.  If  the  projecting  and  re- 
FlG'  3*  entrant  angle  of  the  twin  thus  formed  be  ground 

CALDERON  DOUBLE-  •  .         .  ,     .         i      j  •     •  j     j 

PLATE.  away,  a  plane  plate  xyvw  is  obtained,  divided 

by  the  plane  separating  the  two  pieces  of  calcite  c,  d.  This 
plane  appears  from  above  as  an  extremely  fine  straight  line. 
This  double-plate  is  so  mounted  in  one  of  the  oculars  that  the 
boundary-line  of  the  plate  is  parallel  to  the  chief  section  of  a 
nicol ;  i.e.,  that  both  halves  between  crossed  nicols  show  the 
same  degree  of  extinction. 

A  Fourth  Undulation  Mica  Plate  is  employed  to  determine 
the  character  of  the  double-refraction  in  uniaxial  minerals; 


METHODS  OF  INVESTIGATION.  13 

in  biaxial  minerals,  either  a  plate  of  quartz  about  2  mm.  thick 
and  cut  perpendicular  to  the  optic  axis,  or  a  wedge  of  quartz 
with  one  plane  parallel  to  the  optic  axis  and  the  other  inclined 
at  an  angle  of  about  5°,  is  used. 

In  making  use  of  the  interference-figures  obtained  with  the 
condenser,  to  determine  the  character  of  the  double-refraction 
in  optically-uniaxial  minerals,  the  mica  plate  is  laid  on  the 
tube  so  that  the  plane  of  the  optic  axis  of  the  mica,  generally 
indicated  by  a  mark  on  the  setting,  makes  an  angle  of  45°  with 
the  planes  of  vibration  of  the  nicols. 

In  investigating  optically-biaxial  minerals  the  quartz  wedge 
is  inserted  by  an  opening  in  the  analyzer  so  that  the  chief  axis 
of  the  quartz  forms  an  angle  of  45°  with  the  plane  of  vibration 
of  the  analyzer.  The  interference-figures  of  the  mineral  under 
examination  are  brought,  by  revolving  the  stage,  into  such  a 
position  that  the  plane  of  the  optic  axis  is  at  first  parallel  and 
then  perpendicular  to  the  chief  axis  of  the  quartz  wedge. 

If  but  a  single  quartz  plate  cut  perpendicular  to  the  optic 
axis  is  at  hand,  the  analyzer  must  be  raised  with  one  hand 
from  the  tube  of  the  microscope,  from  which  the  ocular  is 
removed,  so  that  the  quartz  plate  can  be  used  beneath  it, 
care  being  taken  that  both  nicols  remain  exactly  crossed. 
Then  with  the  other  hand  the  quartz  plate  is  turned  a  little 
about  a  horizontal  axis  so  that  the  rays  of  light  must  pass 
through  a  thicker  layer  of  quartz,  and  so  that  the  axis  of 
revolution  is  at  first  parallel  to  the  plane  of  the  optic  axis  of 
the  mineral  and  afterwards  perpendicular  to  it. 

In  order  to  Centre  exactly  any  particular  point  of  an  object 
under  examination,  and  revolve  about  its  own  centre,  so  often 
necessary  in  the  measurement  of  angles  especially,  either  the 
revolving-stage  can  be  moved  in  two  directions  at  right  angles 
to  each  other  (Fig.  I,  m,  11),  or  the  tube  acting  within  a  socket 
can  be  moved  by  two  screws  (Fig.  2,  ;;/;;/,  mi).  There  must  be 


14        DETERMINATION  OF  ROCK-FORMING  MINERALS. 

a  new  centring  of  the  stage  or  tube  for  each  combination  of 
ocular  and  objective. 

If  the  stage  can  be  centred,  one  of  the  centring-screws 
(Fig.  i,  m)  can  serve  at  the  same  time  as  Micrometer.  Each 
revolution  of  this  screw,  the  total  number  being  read  off  fiom 
a  circle  (/)  placed  beside  it,  corresponds  of  course  to  a  definite 
magnitude  of  displacement  of  the  stage,  that  is,  of  the  object 
lying  upon  it ;  e.g.,  in  the  new  microscope  made  by  Fuess,  one 
interval  of  the  micrometer-screw  corresponds  to  a  horizontal 
movement  of  the  stage  of  0.002  mm.  An  ocular-micrometer 
often  accompanies  the  microscopes  instead  of  this  stage- 
micrometer.  Such  a  micrometer  is  made  of  glass,  circular  and 
fitted  to  the  ocular,  with  a  fine  millimetre-scale  engraved  on  it. 

The  method  of  Due  de  Chaulnes  is  best  adapted  to  deter- 
mine the  thickness  of  thin  sections,  i.e.,  the  Index  of  Refraction, 
in  sections  of  minerals  with  parallel  plane  surfaces.  The  mi- 
crometer-screw (Fig.  2,  *g)  moving  the  tube  in  a  vertical  direc- 
tion has  a  graduated  circle  attached,  from  which  the  revolutions 
of  the  screw,  and  therefore  the  extent  of  vertical  movement 
of  the  tube,  can  be  read.  In  Fuess's  instrument,  already  men- 
tioned, the  tube  micrometer-screw  is  divided  into  500  degrees, 
each  of  which  corresponds  to  a  vertical  movement  of  o.ooi  mm. 

The   index  of  refraction   is   determined    according  to   the 

formula  n  —  -j ,  where  d  represents   the   thickness  of  the 

mineral  leaflet,  and  r  the  movement  of  the  tube  which  is  neces- 
sary to  see  a  point  as  clearly  through  the  plate  after  it  is 
introduced  as  before  its  introduction. 

In  order  to  easily  find  a  second  time  such  places  on  the 
preparation  as  may  be  desired,  two  scales  are  placed  at  right 
angles  to  each  other  on  the  stage  (Fig.  I,  c],  which  run  from 
the  centre  of  the  circular  stage  towards  the  o°  and  90°  points 
of  the  outer  graduation  of  the  same  and  are  graduated  into 
whole  or  half  millimetres.  Then  it  is  only  necessary  to  place 


ME  THODS  OF  INVESTIGA  TION.  1 5 

the  object-slide  upon  the  stage  so  that  it  lies  directly  over  the 
two  scales  with  two  of  its  sides  parallel  to  the  marks  of  gradua- 
tion. By  noting  the  numbers  of  these  marks  of  graduation, 
the  position  of  the  preparation  as  to  right  and  left  is  fixed. 
Should  the  object-glass  be  laid  a  second  time  on  the  stage  in 
the  same  position,  the  desired  point  will  fall  within  the  field. 

Finally,  those  microscopes  manufactured  by  Fuess  or  by 
Voigt  &  Hochgesang  are  supplied  with  a  Heating-stage,  with 
thermometer  attached,  to  be  placed  upon  the  circular  revolving- 
stage.  This  can  be  heated  by  an  alcohol-flame  placed  within 
a  mica  chimney,  and  often  does  good  service,  e.g.,  in  determin- 
ing the  fluid  inclosures  in  minerals. 

Different  blendes  are  also  added,  suitable  for  placing  either 
upon  the  ocular,  i.e.,  the  analyzer,  or  of  introduction  in  place 
of  the  polarizer. 

A  heating-apparatus  far  more  to  the  purpose  than  the  one 
just  mentioned,  and  first  suggested  by  Max  Schultze,  is  described 
by  Vogelsang  (Poggend,  Ann.  CXXXVII,  p.  58).  In  it  the 
object  is  warmed  by  a  platinum  wire  heated  by  means  of  a 
galvanic  current.  With  such  an  instrument  a  temperature  of 
200°  C.  can  easily  be  attained,  the  rapidity  of  changes  of  tem- 
perature regulated,  and  any  degree  of  heat  once  reached  con- 
tinued quite  constant. 

The  number  of  different  ocular-  and  objective-lenses-  by 
whose  combination  the  object  can  undergo  a  varying  enlarge- 
ment is  a  matter  of  choice.  In  mineralogico-petrographical 
investigations,  oculars  I,  2,  3,  4  and  objectives  3,  5,  7,  9  of 
Hartnack's  system  generally  suffice.  These  are  usually  con- 
sidered as  the  best,  and  are  supplied  with  the  Fuess  instrument 
as  described. 


1 6         DETERMINATION  OF  ROCK-FORMING  MINERALS. 


A.    Optical   Methods  of  Investigations. 

i.  EXAMINATION  OF  MINERAL  CROSS-SECTIONS  IN  PARAL- 
LEL POLARIZED  LIGHT. 

ROSENBUSCH.     Mikr.  Physiographic,  etc.,  p.  55-107. 
GROTH.     Physikalische  Krystallographie.     Leipzig,  1876. 
E.  KALKOWSKY.     Gr.  Zeitschrift  f.  Kryst.,  IX,  486 

For  observations  in  parallel  polarized  light  both  nicols 
are  exactly  crossed  ;  the  short  diagonals  corresponding  to  the 
direction  of  vibration  in  the  nicols  are  thus  perpendicular  to 
each  other,  total  darkness  of  the  field  following  ;  the  ocular 
and  objective  for  the  desired  magnifying  power  are  inserted 
in  the  tube,  and  the  cross-section  to  be  examined  is  so  placed 
that  on  revolving  the  stage  it  remains  within  the  field,  and  its 
behavior  in  polarized  light  throughout  a  total  revolution  of 
the  stage  noted.  The  gathering-lens,  or  condenser,  above 
the  polarizer  inducing  converging  polarized  light  can  be  left 
in  situ,  as  it  does  not  impede  the  investigations  because  a 
withdrawal  of  the  ocular  is  unnecessary. 

As  is  well  known,  a  discrimination  is  made  between  single- 
and  double-refracting  minerals  ;  the  amorphous  minerals  and 
those  crystallizing  in  the  regular  system  belonging  to  the  first 
class.  The  double-refracting  minerals  are  further  distinguished 
according  to  the  number  of  the  optic  axes  and  of  the  axes  of 
elasticity  as  optically-uniaxial  and  optically  biaxial  minerals. 
Those  minerals  crystallizing  in  the  tetragonal  and  hexagonal 
systems  belong  to  the  optically-uniaxial,  and  those  in  the 
rhombic,  monoclinic,  and  triclinic  systems  to  the  optically- 
biaxial  minerals. 

In  the  following  pages  the  behavior  of  the  minerals  as 
regards  the  different  systems  of  crystallization  to  which  they 
belong  will  be  discussed. 


METHODS  OF  INVESTIGATION-.  I/ 

I.  Single-Refracting  Minerals. 

Amorphous  and  Regular. — If  such  a  mineral  is  placed  under 
the  microscope  with  crossed  nicols,  all  of  its  cross-sections 
remain  perfectly  dark  throughout  a  complete  revolution  of  the 
stage  ;  i.e.,  they  are  isjitrope. 

The  darkness  of  the  field  induced  by  crossing  the  nicols  is 
not  changed  by  introducing  a  section  of  an  amorphous  or 
regularly  crystallizing  mineral,  because  isotrope  bodies  cause 
no  change  in  the  direction  of  vibration  of  the  penetrating 
light,  and  the  elasticity  of  the  ether  in  such  bodies  is  equal  in 
every  direction.  The  index  of  refraction  n  is  constant  for  all 
directions. 

In  the  stauroscope,  with  the  calcite  plate,  no  change  of  the 
interference-figures  occurs  during  a  complete  horizontal  revolu- 
tion, nor  any  change  in  the  shading  of  either  half  of  the 
Calderon  double-plate;  as  they  remain  equally  dark,  the  sepa- 
rating-line  is  invisible. 

A  series  of  amorphous  and  regular  minerals,  including  opal, 
garnet,  analcine,  perowskite,  which  occasionally  appear  as  rock- 
constituents,  show  often  optical  anomalies,  in  that  thin  sections 
of  them  in  parallel  polarized  light  often  brighten  on  revolving 
the  stage.  The  reason  for  these  phenomena  lies  probably  in 
the  internal  tension  produced  during  the  growth  of  the  crystal; 
a  detailed  zonal  structure  is  generally  noticeable  in  such  optical 
anomalies. 

II.  Double-Refracting  Minerals. 

A  mineral  is  double-refracting  when  a  part  of  its  cross- 
section  exhibits  color-phenomena  during  a  complete  revolution 
in  parallel  polarized  light,  i.e.,  shows  polarization-colors.  Such 
cross-sections  become  four  times  colored  and  dark,  the  latter 
always  occurring  in  turning  from  90°  to  90°;  i.e.,  it  extin- 


1 8         DETERMINATION  OF  ROCK-FORMING  MINERALS. 

guishes  the  ray  so  soon  as  one  axis  of  elasticity  coincides  with 
a  chief  section  of  a  nicol.  The  double-refraction  depends 
upon  the  difference  of  the  elasticity  of  the  ether  according  to 
definite  directions  within  these  minerals.  The  color-phenomena 
are  a  consequence  of  the  interference  of  the  light-rays  caused 
by  the  double-refraction,  and  depend  upon  the  magnitude  of 
the  index  of  refraction,  the  direction  of  the  section,  and  the 
thickness  of  the  mineral  leaflet. 

Byuniaxial  minerals,  embracing  the  tetragonal  and  hexago- 
nal systems,  are  understood  those  in  which  the  elasticity  of 
the  ether  differs  in  two  directions,  parallel  or  perpendicular  to 
the  chief  axis.  Here  a  =  the  axis  of  greatest  elasticity,  and  C 
the  least;  and  there  is  but  one  direction  where  no  double- 
refraction  occurs,  viz.,  in  the  direction  of  the  o^tic  axis,  which 
coincides  with  the  chief  axis.  The  index  of  refraction  of  the 
ordinary  ray  ( =  GO)  vibrating  perpendicularly  to  the  optical 
chief  section  (i.e.,  that  plane  which  is  parallel  to  the  optic  axis 
and  perpendicular  to  the  entering  face  of  the  light)  differs  from 
that  (=  f)  of  the  extraordinary  ray  vibrating  in  the  optical 
chief  section.  If  the  chief  axis,  i.e.  the  optic  axis,  coincides 
with  the  axis  of  greatest  elasticity,  c  —  a,  and  oo  >  £,  and  the 
mineral  is  negative ;  if  c  =  C  and  oo  <  e,  the  mineral  is  positive. 
The  greater  the  difference  between  the  indices  of  refraction, 
the  more  powerful  is  the  double-refraction  of  the  mineral. 

A  section  of  a  tetragonal  or  hexagonal  mineral,  cut  perpen- 
dicular to  the  chief  axis  and  parallel  to  oP,  appears  isotrope  in 
parallel  polarized  light  throughout  a  complete  horizontal  revo- 
lution, and  as  one  of  the  single-refracting  minerals;  i.e.,  it  re- 
mains perfectly  darkened.  Sections  parallel  to  the  chief  axis 
and  one  of  the  prismatic  faces  are  generally  rectangular,  and 
between  the  crossed  nicols  are  always  dark  when  one  of  the 
sides  of  the  rectangle,  i.e.,  one  of  the  planes  of  cleavage  paral- 
lel to  the  chief  axis,  is  parallel  to  one  of  the  chief  sections  of  a 
nicol  or  an  arm  of  the  cross-wires.  This  occurs  four  times 


METHODS  OF  INVESTIGATION. 


during  one  complete  revolution.  The  longitudinal  section  is 
then  said  to  EXTINGUISH  PARALLEL  to  the  crystallographicaxes. 

Fig.  4  gives  a  clear  idea  of  this  parallel  extinction  in  an 
optically-uniaxial  mineral  cross-section  abed,  c  is  the  chief 
axis,  and  viv  and  xy  are  the  cross- 
sections  of  both  crossed  nicols, 
whose  optical  chief  sections  coin- 
cide with  the  short  diagonals  of 
the  rhombic  transverse  section. 

So  soon  as  the  chief  axis,  i.e., 
one  of  the  sides,  forms  any  angle 
with  the  nicol  chief  section  and  the 
cross-wires,  the  longitudinal  sec- 
tion shows  the  polarization-colors. 

Sections    inclined    tO    the    Chief  FIG.  ^—PARALLEL  EXTINCTION. 

axis,  e.g.,  parallel  to  a  pyramidal  plane,  of  course  always  ex- 
tinguish parallel  to  the  chief  axis,  but  not  always  parallel  to 
the  sides.  Thus  a  triangular  or  pentagonal  cross-section  ex- 
tinguishes parallel  to  one  of  the  sides,  as  the  chief  axis  in 
such  sections  is  perpendicular  to  the  direction  of  one  of  these 
sides,  while  a  rhombic  cross-section  will  extinguish  parallel  to 
the  diagonals  of  the  figure. 

The  behavior  of  various  cross-sections  of  a  uniaxial  crystal 
in  parallel  polarized  light  can  be  easily  demonstrated  on  a  glass 
crystal  model  in  which  the  chief  axis  is  marked,  if  one  will 
always  bear  in  mind  that  the  extinction  occurs  parallel  to 
the  chief  axis. 

In  the  stauro-microscope  (with  calcite  plate)  transverse 
sections  of  optically-uniaxial  minerals  always  show  the  calcite 
interference-figures.  In  longitudinal  sections  they  are  undis- 
turbed only  when  the  chief  axis  or  one  of  the  contour-lines  of 
the  crystal  parallel  to  it  coincides  with  one  of  the  arms  of  the 
cross-wires  already  in  conjunction  with  the  nicol  chief  sections 
in  the  microscope. 


20         DETERMINATION  OF  ROCK-FORMING  MINERALS. 

Transverse  sections  behave  like  isotrope  cross-sections  when 
examined  with  the  Calderon  double-plate.  Longitudinal  sec- 
tions always  induce  a  different  shading  of  both  halves  of  the 
plate  when  the  chief  axis  is  not  parallel  to  the  principal  direc- 
tion of  vibration  of  the  nicol,  the  arms  of  the  cross-wires,  or 
the  line  of  junction  in  the  Calderon  plate,  three  objects  which 
are  exactly  parallel  to  each  other  in  the  microscope.  If  the 
chief  axis  is  parallel  to  the  line  of  junction,  both  halves  of  the 
plate  are  equally  dark  with  crossed  nicols;  if, this  is  not  the 
case,  then  both  halves  are  unequally  shaded,  the  one  dark  and 
the  other  light,  or  both  are  equally  clear. 

It  is  possible  to  determine  whether  a  mineral  under  exam- 
ination belongs  to  the  tetragonal  or  hexagonal  system  only 
from  the  character  of  the  contour  of  the  section  cut  at  right 
angles  to  the  chief  axis.  If  it  is  square  or  octagonal  it  belongs 
to  the  tetragonal ;  if  hexagonal  or  dihexagonal,  it  belongs  to 
the  hexagonal  system. 

In  the  optically-biaxial  minerals  there  are  two  directions 
wherein  no  double-refraction  takes  place,  i.e.,  there  are  two 
optic  axes ;  and  further,  we  assume  three  axes  of  elasticity  at 
right  angles  to  each  other,  i.e.,  three  directions  in  which  the 
elasticity  of  the  light-ether  differs.  The  direction  of  the  great- 
est elasticity  is  designated  by  a,  that  of  middle  value  by  b,  and 
that  of  the  least  by  c. 

The  optic  axes  do  not  coincide  with  the  crystallographic 
axes,  and  form  an  angle  with  each  other.  The  line  dividing 
equally  the  acute  angle  is  called  the  first  middle  line,  or  acute 
bisectrix;  the  line  bisecting  the  more  obtuse  angle,  the  second 
middle  line,  or  obtuse  bisectrix.  The  optic  axes  and  both  middle 
lines  lie  in  a  single  plane,  THE  PLANE  OF  THE  OPTIC  AXES 
(A.P.) ;  the  optic  normal  lies  perpendicular  to  the  plane  of  the 
optic  axes.  The  axis  of  elasticity  of  middle  value  (b)  always 
coincides  with  the  optic  normal,  while  the  axes  of  greatest  and 
least  elasticity  coincide  with  either  the  first  or  the  second 


METHODS  OF  INVESTIGATION.  21 

middle  line.  If  a  =  I.  M.,  then  c  =  2.  M,,  and  the  mineral  is 
negative;  if  c  =  i.  M.,  and  a  =  2.  M.,  the  mineral  is  positive. 

There  are  three  different  indices  of  refraction,  a,  fi,  y, 
corresponding  to  these  three  axes  of  elasticity. 

Minerals  crystallizing  in  the  rhombic,  monoclinic,  and  tri- 
clinic  systems  belong  to  the  double-refracting  minerals. 

Rhombic  Minerals. — In  these  minerals  the  three  axes  of 
elasticity  a  >  b  >  C  coincide  with  the  three  crystallographic 
axes  a,  b,  c'  \  a  does  not  always  equal  a,  etc.,  yet  each  of  the 
crystallographic  axes  can  coincide  with  each  of  the  axes  of 
elasticity,  a  and  c  are  always  middle  lines,  and  the  plane  of 
the  optic  axes  (AP)  is  always  parallel  to  one  of  the  three 
pinacoids.  The  following  cases  may  therefore  occur: 

If  AP  ||  oP,  then  d  =  a.  b  =  C )    ,  _ 

a  =  t,6  =  a  f  c 

If  AP  ||  oo  ^oo  ,  then  c'  ~  a,  d  —  c  f  -, 

.  r  o   •=.  0  ; 

c'  =  c,  d <  =  a  1 

IiAP\\  oo  P  oo  ,  then  c'  —  a,  b  =  c  )    . 

-  >  a  =  b. 

c1  —  c,  b  —  a  I 

Figs.  5  to  8  serve  as  examples  of  these  cases.  These  are 
schematic  representations  of  the  optic  orientation  of  rhombic 
augite  and  hornblende  in  sections  parallel  to  the  plane  of  the 
optic  axes.  A  and  B  represent  the  two  optic  axes;  the  middle 
lines  or  axes'  of  elasticity  are  designated  by  German,  the  crys- 
tallographic axes  by  italic  letters. 

Cross-sections  parallel  to  the  three  planes  of  the  pinacoids, 
in  general  of  rectangular  figure,  have  a  parallel  extinction,  i.e., 
are  dark  between  crossed  nicols  only  when  one  of  the  sides  of 
the  rectangle  or  one  of  the  pinacoidal  cleavage-fissures  is 
parallel  to  a  chief  section  of  a  nicol.  Darkness  follows  so  soon 
as  one  of  the  crystallographical  axes  coincides  with  a  nicol 
chief  section.  This  occurs  four  times  in  a  complete  revolution, 


22         DETERMINATION  OF  ROCK-FORMING  MINERALS. 

just  as  with  the  longitudinal  sections  of  the  uniaxial  crystals. 
The  rhombic  minerals  can,  however,  be  distinguished  from 
them  in  parallel  polarized  light,  in  that  the  sections  parallel  to 
oP  are  not  isotrope  as  in  the  uniaxial  minerals. 

Only   those   sections   of  rhombic   minerals  which  are   cut 


FIG.  5.— HYPERSTHENE. 

||  00  P  03. 


FIG.  7.— ENSTATITE  AND  BRONZITE. 

||  00/00. 


FIG.  8. — ANTHOPHYLLITE. 

II  00/00. 


exactly  at  right  angles  to  one  of  the  two  optic  axes  remain  per- 
fectly dark  throughout  a  complete  revolution,  i.e.,  are  isotrope. 
Such  sections  are  parallel  to  Poo,  ^oo,  or  a  prismatic  face, 
according  to  the  position  of  the  optic  axes.  It  is  self-evident 
that  such  isotrope  sections  are  more  rare  in  rhombic  than  in 
optically-uniaxial  crystals,  and  have,  moreover,  no  such  regular 
forms. 


METHODS  OF  INVESTIGATION.  2$ 

Just  as  in  the  pinacoidal  sections,  i.e.,  from  the  zones 
oP  :  oo  .Poo  and  oP  :  co/^oo,  so  all  longitudinal  sections  parallel 
to  the  vertical  axis  (c')  from  the  zone  ooPoo  :  oo/^oo  extinguish 
parallel  to  the  sides  or  one  of  the  cleavage-fissures  parallel  to 
the  vertical  axis.  Symmetrical  sections  inclined  to  the  ver- 
tical axis,  not  belonging  to  any  of  the  above  zones,  do  not 
extinguish  for  the  most  part  according  to  their  axial  figures. 

When  examined  with  the  stauroscope,  the  calcite  inter- 
ference-figures, i.e.,  the  darkening  of  the  Calderon  double-plate, 
appear  undisturbed  only  when  one  of  the  crystallographic  axes 
coincides  with  a  nicol  chief  section ;  isotrope  sections,  of  course, 
exert  no  action  on  either  plate  during  a  complete  revolution. 

Monoclinic  Minerals. — In  the  monoclinic  system  only  the 
orthodiagonal  axis  b  coincides  with  one  of  the  axes  of  elas- 
ticity ;  both  of  the  remaining  axes  of  elasticity  form  an  angle 
with  the  crystallographic  axes  a  and  c'.  The  plane  of  the 
optic  axes  is  either  parallel  or  at  right  angles  to  the  plane  of 
symmetry  ooPoo.  In  the  monoclinic  minerals  there  are  the 
following  possibilities  for  optical  orientation : 

If  AP\\  co  Poo,  then  I.  M.  =  c  )  i       fc. 
or  i.M.  =  a) 

and  c  and  a  are  inclined  to  c'  and  a. 

If,  on  the  contrary,  AP±  oo  jPoo,  then  I.  M.  =  b  =  0, 

1.  M.  =  b  =  c; 
or  2.  M.  =  b  —  a, 

2.  M.  =  b  =  c. 

In  this  case  b  and  c,  or  b  and  a,  are  inclined  to  c'  and  a. 

In  Figs.  9  to  14  are  given  schematic  representations  of 
several  rock-forming  monoclinic  minerals.  The  cross-sections 
are  parallel  to  the  plane  of  the  optic  axes.  A  and  B  are  the 


24         DETERMINATION  OF  ROCK-FORMING  MINERALS. 


FIG.  g. — HORNBLENDE. 

II  00  P 00. 

(After  Fouque".) 


FIG.  io.— AUGITE. 

HooPoo. 
(After  Fouqu^.) 


FlG.  II.— WOLLASTONITE. 
II  oo  Poo. 

(After  Fouqud.) 


FIG.  12.— EPIDOTE. 

II  oo  Poo. 

(After  Fouque".) 


METHODS  OF  INVESTIGATION 


. 


optic  axes,  d  and  C  middle  lines,  and  c  the  vertical 
titanite,  Fig.  13,  there  is  shown,  in  addition,  the  dispersion 
of  the  optic  axes  v  <  p ;  in  orthoclase,  Fig.  14,  attention  is 
called  to  the  case  where  the  plane  of  the  optic  axes  is 
1  oo^oo.  A^B1  are  the  optic  axes  where  AP\\  oo jPoo,  A^B, 


FIG.  13  — TITANITE. 

HcojPoo. 
(After  Fouque.) 


FIG.  14.— ORTHOCLASE. 

II  oo  f> oo. 
(After  Fouque.) 


where  AP  is  J_  ooPoo;  in  both  cases  a  is  at  an  angle  of  5°  to 
the  edge  oP  :  oo  Poc. 

As  a  consequence  of  this  inclination  of  the  axes  of  elas- 
ticity to  the  crystallographic  axes,  longitudinal  sections  are 
not  darkened  during  a  complete  revolution  whenever  the 
crystallographic  axes  or  the  cleavage-fissures  parallel  "to  these 
coincide  with  a  nicol  chief  section,  as  is  the  case  with  rhom- 
bic minerals ;  but  in  many  cases  extinction  (i.e.,  the  section 
becomes  dark  under  crossed  nicols)  first  takes  place  when  the 
crystallographic  axes  are  inclined  to  the  nicol  chief  section; 
i.e.,  it  extinguislies  obliquely. 

Fig.  15   represents   the  oblique  extinction  of   an    optically- 


26 


DETERMINATION  OF  ROCK-FORMING  MINERALS. 


biaxial  crystal   cross-section,  abed,    wherein  C  represents   the 

vertical  axis,  and  vw  and  xy  are 
again  the  nicol  cross-sections. 
The  crystal  cross-section  is  in 
the  position  where  it  completely 
extinguishes  the  ray;  the  incli- 
nation of  the  axis  of  elasticity 
lying  in  the  direction  xy  to  the 
vertical  axis  is  50°  in  this  case. 

Extinction  always  follows,  as 
is  well  known,  whenever  one 
FIG.  15.— OBLIQUE  EXTINCTION.  axis  of  elasticity  coincides  with 
a  nicol  chief  section ;  in  the  monoclinic  system,  however,  two 
axes  of  elasticity  are  always  inclined  to  the  crystallographic 
axes.  This  angle  of  inclination  is  exceedingly  characteristic 
for  the  various  monoclinic  crystals  (comp.  Figs.  9  to  14),  and 
can  be  readily  determined  in  parallel  polarized  light.  As  a 
consequence  of  the  optical  orientation  this  oblique  extinction 
can  be  accurately  determined  only  in  those  sections  parallel 
to  the  plane  of  symmetry,  oopoo  (comp.  Fig.  15).  A  cleavage- 
fissure  parallel  to  the  chief  axis,  or  one  of  the  edges  parallel 
to  it,  is  placed  in  position  parallel  to  an  arm  of  the  cross-wires 
(i.e.,  a  nicol  chief  section),  and  the  degrees  read  on  the  circle 
of  the  stage.  In  this  position,  between  crossed  nicols,  the 
cross-section  is  colored.  The  stage  is  then  revolved  until  the 
cross-section  is  perfectly  darkened.  The  number  of  degrees 
through  which  the  stage  must  be  revolved  to  cause  total  dark- 
ness gives  the  angle  of  inclination  of  one  axis  of  elasticity  to 
the  vertical  axis,  the  "  angle  of  extinction  ;"  e.g.,  on  augite 
the  inclination  C  :  c  =  38°,  therefore  a  :  c  —  52°.  This  angle 
which  one  of  the  axes  of  elasticity  forms  with  the  vertical 
axis  of  course  equals  that  which  the  other  axis  of  elasticity 
makes  with  the  normal  to  co  P  co.  The  angle  of  extinction  can 
also  be  measured  in  relation  to  another  known  edge  in  the 


METHODS  OF  INVESTIGATION. 

section   ||   oojPco;  e.g.,  to  the  edge  oP :  oojPoo,  i.e.,  as 

the  same  inclination  as  ^,  the  angle  of  inclination  of  the  other 

axis  of  elasticity  to  the  clinodiagonal. 

The  application  of  the  stauroscope  is  therefore  clear  from 
what  has  just  been  stated.  This  is  used,  as  it  is  very  difficult 
to  determine  with  the  eye  alone  the  exact  point  of  maximum 
darkness ;  with  the  aid  of  an  exceedingly  sensitive  Calderon 
double  plate,  however,  this  is  possible  with  accuracy  to  within 
some  few  minutes;  it  is  therefore  peculiarly  adapted  to  the 
more  exact  determination  of  the  position  of  the  plane  of  the 
axes  of  elasticity.  Equality  of  shading  in  the  double.-plate  of 
course  follows  when  an  axis  of  elasticity  is  parallel  to  the  line 
of  junction  in  the  plate. 

All  sections  of  monoclinic  crystals  from  the  zone  oP : 
oo  P  co  extinguish  parallel,  as  in  these  the  orthodiagonal 
always  coincides  with  one  of  the  axes  of  elasticity;  extinc- 
tion follows,  therefore,  always  when  one  of  the  edges  parallel 
to  the  vertical  axis  or  one  of  the  cleavage-fissures  parallel  to 
this  coincides  with  a  chief  section  of  a  nicol.  The  shading  of 
the  Calderon  double-plate  will  therefore  be  undisturbed  only 
when  the  orthodiagonal  coincides  with  a  nicol  chief  section, 
i.e.,  the  line  of  junction. 

Sections  from  the  zones  oP  '.&£>&>  and  oo  P  oo  :  co  P  oo 
always  extinguisli  at  an  angle.  The  angle  of  extinction  finally 
reaches  o°  when  the  section  is  parallel  to  oP  or  oo^co. 

Thus,  according  to  Michel  Levy,  the  value  of  the  oblique 
extinction  varies  in  augite  and  hornblende  with  the  direction 
of  the  section  iii  the  following  manner : 


28        DETERMINATION  OF  ROCK-FORMING  MINERALS. 


Direction  of  the 
section  in  the  zone. 

Augfite. 
For  2v  =  58°  59'. 

Hornblende. 
For  -2V  =  79°  24'. 

oP  :  co  Poo 

;  =  :=f2:#l  Maximum. 

In  sections  parallel  to  cof><x>J 
a  :  a  =  22°  55';     with    more 
acute   inclination    of   the   sec- 
tion the  value  increases  and 
reaches   its  maximum  on  the 
plane   which  makes  an  angle 
of  67°   14'    6"   with     co  f  oo  ; 
it  then  lessens    and   becomes 
o°  in  sections  parallel  to  oP. 

Maximum  of  29°  58'  to  14° 
58'  parallel  to  00^*00,  accord- 
ing to  the  species  of  horn- 
blende, then  decreases  and 
becomes  o°  parallel  to  oP. 

CQ-^  co   ;  co  f  oo 

Maximum  of   extinction   ob 
liquity  on  coPoo,   c  :  c  =  38° 
44'.     According    to    the    in- 
clination   towards  co/oo  the 
angle  decreases  and  becomes 
o°  parallel  co^  oo. 

Minimum  parallel  QOJ?  co,  be- 
tween 15°  (for  hornblende) 
and  o°  (actinolite);  increases 
and  reaches  the  maximum 
(actinolite  =  15°  15'  20")  in 
the  plane  which  forms  an 
angle  of  38°  18'  25"  with 
oo  f  oo  ;  then  decreases  and 
equals  o°  parallel  oo  P  oo. 

oP  :  00/5 


All  sections  possess  parallel  extinction 


'WJ5' 


Triclinic  Minerals. — In  the  triclinic  minerals  no  one  of  the 

^ CP  three  axes  of  elasticity  coincides  with 

the  crystallographic  axes. 
e  Fig.  16  is  an  example  of  the  opti- 

cal orientation  of  a  triclinic  rock- 
>JP»  forming  mineral,  a,  b,  and  c  are  the 
three  axes  of  elasticity ;  the  angle  of 
inclination  of  c  to  the  vertical  axis 
in  disthene  is  30°  measured  in  sections 
parallel  to  ooPrc;  a  is  exactly  per- 
pendicular tO  OQ^CC. 

All  sections,  therefore,  parallel  to 
the  three  pinacoidal  faces  have  the  oblique  extinction. 
The  obliquity  of  extinction  to  the  faces  oP  and  oo  P  oo 


FIG.  16.— DISTHENE. 

HOODOO. 

(After  Fouqud.) 


METHODS  OF  INVESTIGATION  2$ 

is  known  in  most  of  the  rock-forming  minerals,  and  gives, 
therefore,  an  excellent  means  of  determining  the  minerals 
of  this  system.  In  the  thin  sections  one  can  in  most  cases 
determine  from  the  shape  of  the  cross-section  whether  it  is 
parallel  to  one  of  these  pinacoids.  If  now  an  oblique  extinc- 
tion is  proved  on  both  of  these  pinacoids,  it  is  sufficient  for 
assignment  of  the  mineral  to  the  triclinic  system,  as  in  the 
monoclinic  system  the  oblique  extinction  obtains  only  parallel 
to  the  plane  <x>jPo>.  Exact  measurement  of  the  extinction- 
obliquity  must,  however,  be  made  on  cleavage-lamellae  parallel 
oo/^co  and  oP. 

In  the  stauroscope  the  calcite  interference -figures,  i.e.,  the 
shading  of  the  Calderon  double-plate,  will  be  disturbed  when- 
ever one  of  the  crystallographic  axes  or  a  cleavage  line  or  edge 
parallel  to  them  is  parallel  to  a  nicol  chief  section. 


2.  EXAMINATION  OF    MINERALS   IN    CONVERGENT 
POLARIZED  LIGHT. 

In  order  to  produce  convergent  polarized  light  the  con- 
denser is  placed  above  the  polarizer,  and,  after  the  cross- 
section  has  been  adjusted  and  centred  in  the  microscope,  the 
ocular  is  removed  and  the  nicols  crossed.  If  the  cross-section 
is  very  small,  and  high  magnifying  powers  must  be  used, 
thereby  decreasing  the  interference-figures,  the  Bertrand  lens, 
for  the  necessary  enlargement,  is  inserted  within  the  tube  in 
place  of  the  ocular,  and  the  nicols  of  course  are  again  crossed. 

The  interference-figures  observed  with  the  condenser  in 
different  sections  of  the  double-refracting  minerals  are  exactly 
the  same  as  those  obtained  on  such  sections  with  the  Norrem- 
berg  instrument.  The  interference-figures,  however,  are  not 
so  clear  and  large  in  the  microscope,  as  the  mineral  cross- 
sections  are  very  small,  and  in  the  slides  are  exceedingly 


30         DETERMINATION  OF  ROCK-FORMING  MINERALS. 

thin.  The  great  advantage  gained  through  the  application 
of  the  condenser  to  microscopical  petrography,  as  first  recom- 
mended by  Lasaulx  and  Bertrand,  is  evident ;  e.g.,  we  can 
determine  whether  a  mineral  is  a  single-refracting,  optically 
uniaxial  or  biaxial,  if  but  a  single  isotrope  cross  section  of  the 
mineral  is  at  hand.  The  following  observations  will  demon- 
strate this.  Of  course  the  examination  in  parallel  polarized 
light  must  always  precede  that  in  convergent  light. 

Clear  interference-figures  can  be  obtained  on  using  objec- 
tive 9,  Hartnack,  and  the  Bertrand  magnifying-lens  in  cor*, 
vergent  light,  if  the  mineral  cross-section  is  not  less  than  0,05 
mm. ;  if  the  cross-sections  are  less,  their  determination  in  con- 
vergent light  is  impossible  in  most  cases.  In  such  cases  the 
examination  in  parallel  polarized  light  is  all  the  more  important. 
The  behavior  of  mineral  cross-sections  in  convergent  light  for 
the  different  systems  of  crystallization  is  the  following  : 

Regular  and  Amorphous  Minerals. — The  amorphous  miner- 
als and  those  crystallizing  in  the  regular  system  remain  dark 
throughout  a  complete  revolution  in  all  cross-sections,  and 
show  no  interference-phenomena. 

Optically-TTniaxial  Minerals  (Fig.  17,  I  and  II). — The  iso- 
trope transverse  sections  of  tetragonal  and  hexagonal  miner- 
als show,  in  case  the  section  is  exactly  at  right  angles  to  the 
chief  axis  (Fig.  17,  I),  a  fixed  dark  interference-cross  with 
several  colored  concentric  rings.  The  presence  and  number  of 
the  rings  in  the  cross  section  depend  on  its  thickness  and  the 
power  of  double-refraction  of  the  mineral.  If  the  section  is 
not  exactly  at  right  angles  to  the  chief  axis,  as  is  evident  in 
ordinary  light  from  the  irregular  transverse  section  (e.g.,  dis- 
torted rectangles  or  hexagons),  or  the  confirmation  of  an 
imperfect  depolarization  in  parallel  polarized  light,  the  inter- 
ference-cross in  convergent  polarized  light  remains  undisturbed 
throughout  a  complete  horizontal  revolution  ;  i.e.,  it  does  not 
open,  but  moves  according  to  the  lesser  or  greater  inclination 


V 


METHODS  OF  INVESTIGATION.  31 

of  the  section  to  the  chief  axis  either  within  the  field  or 
without  on  the  circumference,  and  in  the  same  direction  as 
the  stage  is  revolved. 

If  the  section  is  so  inclined  that  the  axial  point  of  the  inter- 
ference-figure falls  without  the  field  of  the  microscope  (Fig.  17, 
II),  it  will  not  appear  in  parallel  light  as  isotrope  (show  polari- 
zation-colors and  become  darkened  four  times  during  a  revolu- 
tion) ;  in  this  case,  by  revolving  the  stage  from  90°  to  90°  only 
one  part  of  the  interference-cross  will  appear  as  a  straight  black 
cloud  in  the  field.  The  cloud  moves,  during  a  revolution  of 
the  stage  through  90°,  from  one  side  of  the  microscope-stage, 
i.e.,  the  field  of  the  microscope,  to  the  other  in  the  same  plane. 
As  will  be  shown  later,  similar  pictures  will  be  obtained  in  sec- 
tions of  optically-biaxial  minerals  which  are  cut  at  right  angles 
to  one  of  the  optic  axes,  yet  the  black  cloud  in  these  moves 
about  an  axial  point  situate  within. 

For  the  Determination  of  the  Character  of  the  Double  Refrac- 
tion (in  sections  at  right  angles  to  the  chief  axis  a  fourth  undu- 
lation mica  plate  is  most  advantageously  employed.  As  already 
stated,  this  is  laid  on  the  tube  from  which  the  ocular  has  been 
removed,  and  the  analyzer  placed  upon  it,  with  the  nicols 
crossed,  and  with  the  plane  of  the  optic  axes  of  the  mica  in- 
clined at  an  angle  of  45°  to  a  nicol  chief  section.  The  black 
interference  cross  of  a  uniaxial  crystal  diminishes  until  only 
two  dark  points  remain  and  the  colored  rings  are  disturbed. 

If  the  two  dark  points  are  so  situated  thaj:  the  line  joining 
them  is  perpendicular  to  the  plane  of  the  "bptitTaxes  of  the 
mica  (generally  indicated  by  a  mark  on  the  plate),  then  the 
mineral  under  examination  is  optically  positive ;  if  the  joining 
line  of  both  the  dark  points  concides  with  the  direction  of  the 
axial  plane  of  the  mica,  the  mineral  is  optically  negative. 

Optically-Biaxial  Minerals  (Fig.  17,  III,  IV,  and  V).— If 
an  optically-biaxial  mineral  be  cut  at  right  angles  to  one  of  the 
middle  lines  bisecting  the  angle  which  the  two  optic  axes  make 


32         DETERMINATION  OF  ROCK-FORMING  MINERALS. 
OPTICALLY-UNIAXIAL  CRYSTALS. 


OPTICALLY-BIAXIAL   CRYSTALS. 


FlG.  17. — INTERFERENCE-FIGURES  OF  DOUBLE-REFRACTING  MINERALS  ON  USING  THE  CONDENSER 
IN  THE  POLARIZATION-MICROSCOPE.    (After  Fouqu^.) 

$  is  the  angle  which  a  vertical  plane  passing  through  an  optic  axis  A  forms  with  the  optic 
chief  section  of  the  polarizer. 


METHODS  OF  INVESTIGATION.  33 

with  each  other  (Fig.  17,  V),  and  be  examined  in  convergent 
polarized  light,  an  interference-figure  is  seen,  in  case  the  plane 
of  the  optic  axes  coincides  with  a  nicol  chief  section,  which 
is  formed  from  two  closed  systems  of  curvature  correspond- 
ing to  the  two  axial  points ;  these  in  turn  are  surrounded 
by  a  larger  system  of  curvatures,  the  lemniscates,  and  are  trav- 
ersed by  a  black  cross  of  which  one  arm,  the  narrower,  passes 
through  the  two  axial  points  and  thus  shows  the  position  of 
the  plane  of  the  optic  axes,  and  whose  second  arm,  much 
broader,  is  at  right  angles  to  it. 

The  number  of  the  colored  curves  depends,  again,  on  the 
thickness  of  the  mineral  leaflet ;  if  this  is  very  thin,  as  may  be 
expected  in  rock-sections,  only  the  black  cross  is  visible,  thus 
resembling  the  interference-figure  of  optically-uniaxial  crystals. 
The  difference  is  immediately  seen  on  revolving  the  mineral 
section  on  the  stage  (Fig.  17,  V,  <p  >  45°);  in  the  optically- 
biaxial  minerals  the  cross  does  not  remain  fixed,  but  opens 
and  divides  into  two  hyperbolas  which  move  about  either  axial 
point  and  by  revolving  90°  again  close  into  the  cross. 

The  distance  between  the  two  points,  or  the  hyperbolas 
passing  through  them,  gives  both  the  position  of  the  plane  of 
the  optic  axes|  and  the  magnitude  of  the  axial  angle ;  if  this 
angle  is  large,  then  each  of  the  hyperbolas  lies  without  the 
field,  so  soon  as  the  plane  of  the  axes  forms  an  angle  of  45° 
with  a  nicol  chief  section  (Fig.  17,  V,  cp  —  45°).  It  can  gen- 
erally be  determined  from  the  proximate  estimation  of  the 
magnitude  of  the  axial  angle  whether  the  section  is  made  per- 
pendicular to  the  first  or  second  middle  line.  There  are  cases, 
as  in  the  rhombic  pyroxenes,  where  the  acute  axial  angle  differs 
but  little  from  the  obtuse  ;  in  such  cases  it  is  impossible  to  de- 
termine*by  the  microscope  which  axes  of  elasticity  coincide 
with  the  first  and  second  middle  lines. 

If  it  is  known  whether  the  section  is  at  right  angles  to  the 
first  or  second  middle  line,  then  it  can  be  determined  which  of 


34         DETERMINATION  OF  ROCK-FORMING  MINERALS. 

the  axes  of  elasticity  a  or  c  coincides  with  the  same,  i.e.,  the 
optical  orientation.  If  the  axial  angle  is  very  small,  the  inter- 
ference-figure will  be  similar  to  the  optically-uniaxial  mineral 
and  the  cross  apparently  remains  closed. 

The  Determination  of  the  Character  of  Double-refraction  in  the 
optically-biaxial  crystals  is  effected  in  the  following  manner: 
The  axial  figure  is  placed  in  such  a  position  that  the  plane  of  the 
optic  axis  is  at  an  angle  of  45°  with  a  nicol  chief  section,  i.e., 
the  cross  seems  merged  into  the  hyperbolas  ;  the  quartz  plate 
described  on  page  10,  or  the  quartz  wedge,  is  so  used  beneath 
the  analyzer  that  the  axis  of  revolution  of  the  quartz  plate  or 
quartz  wedge  is  at  first  parallel  and  then  perpendicular  to  the 
plane  of  the  optic  axes.  In  any  case,  a  change  of  the  inter- 
ference-figure is  visible  on  revolving  the  quartz  plate  or  on 
pushing  in  the  quartz  wedge  ;  the  inner  rings  move  from  the 
circumference  of  the  field  towards  the  centre,  the  outer  lem- 
niscates,  on  the  other  hand,  in  the  opposite  direction.  If  this 
enlargement  and  movement  of  the  rings  occur  when  the  axis 
of  revolution  of  the  quartz  plate  or  the  quartz  wedge  is  perpen- 
dicular to  the  plane  of  the  optic  axis,  the  mineral  is  positive 
double-refracting  ;  under  reversed  conditions,  negative. 

•  If  the  mineral  was  proved  positive  double-refracting  on  sec- 
tions at  right  angles  to  the  first  middle  line,  then  the  axis  of 
the  least  elasticity  coincides  with  it  and  the  plan  is  the  fol- 
lowing: 

First  middle  line  =  c  (positive) ; 
Second  middle  line  =  a; 
Optic  normals  always  =  b. 

The  reverse  is  true  in  case  the  second  middle  line  is  positive : 

First  middle  line  —  a  (negative) ; 
Second  middle  line  =  c; 
Optic  normals  —  fc. 


METHODS  OF  INVESTIGATION.  35 

Sections  at  right  angles  to  one  of  the  two  optic  axes  appear 
as  isotrope  in  parallel  polarized  light,  and  show  in  convergent 
polarized  light  a  spherical  or  elliptical  colored  ring-system 
traversed  by  a  dark  cloud  (Fig.  17,  III).  If  the  section  is 
exactly  at  right  angles  to  the  optic  axis,  on  revolving  the  prep- 
aration the  cloud  moves  in  a  contrary  direction  about  the  axial 
point  lying  in  the  centre  of  the  ring-system  ;  on  sections  more 
or  less  inclined  to  the  optic  axes  (Fig.  17,  IV)  a  movement  of 
the  whole  axis-figure  is  observed  concordant  with  the  revolv- 
ing of  the  object-stage. 

If  the  section  is  so  oblique  to  the  optic  axis  (Fig.  17,  IV) 
that  the  axial  point  falls  without  the  field,  only  a  part  of  the 
cloud  ever  lies  in  the  centre  of  the  field  on  revolving  from  90°  to 
90°,  just  as  with  the  optically-uniaxial  minerals  cut  inclined  to 
the  axis  ;  the  difference  consists,  however,  in  the  movement  of 
the  cloud  itself  about  the  axis-point  in  the  direction  opposite 
to  that  of  the  revolving-stage.  Sections  parallel  to  the  plane 
of  the  optic  axes,  at  right  angles  to  B,  show  no  interference- 
figures  in  convergent  polarized  light,  become  colored  as  in 
parallel  polarized  light,  and  appear  dark  whenever  an  axis  of 
elasticity  coincides  with  a  nicol  chief  section. 

Rhombic  Minerals. — Sections  at  right  angles  to  the  crystallo- 
graphic  axes,  consequently  parallel  to  the  pinacoidal  planes, 
show  perfectly  the  optical  orientation.  According  to  the  posi- 
tion of  the  plane  of  the  optic  axes  (see  page  22),  either  the 
vertical  axis,  the  brachy-  or  macro-diagonal  will  be  the  first 
middle  line.  One  of  the  pinacoidal  sections  will  show  perpen- 
dicular to  it  the  first  middle  line  with  the  smaller  angle  of  the 
optic  axes,  a  second  the  appearance  of  the  second  middle  line 
with  the  larger  axial  angle,  and  the  third  parallel  to  the  plane  of 
the  axes  will  show  no  interference-figures.  The  transverse  sec- 
tions are  the  most  favorable  (at  right  angles  to  c'}\  as  on  the  one 
hand  but  few  of  the  rock-forming  minerals,  e.g.,  oliv-ine,  have 
the  axial  planes  parallel  oP,  consequently  in  these,  at  any  rate, 


36          DETERMINATION  OF  ROCK-FORMING  MINERALS. 

an  interference-figure  is  seen ;  and  on  the  other  hand,  as  the 
predominating  cleavage  is  prismatic  or  pinacoidal,  it  can  be 
controlled  as  to  whether  the  section  is  made  exactly  at  right 
angles  to  the  vertical  axis. 

As  a  consequence  of  the  dispersion  of  the  optic  axes  the  in- 
terference-figure develops  in  white  light  a  varying  color-distri- 
bution according  as  the  axial  angle  for  red  is  greater  or  smaller 

than  for  blue  (p  ^  v) ;  in  the  rhombic  system  the  distribution 

is  symmetrical  to  the  middle  lines.  Where  p  >  ^>,  in  the  posi- 
tion :  axial  plane  parallel  to  the  nicol  chief  section,  the  inner 
closed  curves  are  blue  on  the  inner  limb,  and  red  on  the  outer ; 
in  the  position:  axial  plane  inclined  45°  to  the  nicol  chief  sec- 
tion, the  hyperbolas  become  red  on  the  inner,  the  convex  sur- 
face, and  blue  on  the  outer,  the  concave  surface.  Where  p  <  v 
the  reverse  holds  true.  The  phenomena  of  dispersion,  when 
not  too  weak,  can  be  studied  in  convergent  light  very  well  in 
the  rock-constituents,  e.g.,  zoisite,  etc.  Often  the  simple  obser- 
vation of  an  hyperbola  in  relation  to  the  colored  edges  suffices 
for  the  determination  of  the  form  of  axial  dispersion ;  it  is  not 
absolutely  necessary,  therefore,  that  the  sections  should  be  at 
right  angles  to  the  middle  lines. 

Monoclinic  Minerals. — If  the  plane  of  the  optic  axes  in  mono- 
clinic  minerals  is  parallel  ooPoo,  the  sections  at  right  angles  to 
the  vertical  axis  and  parallel  oo  Pco  will  not  show  a  perpendicu- 
lar development  of  a  middle  line,  as  is  the  case  with  the  cor- 
responding pinacoidal  sections  of  rhombic  crystals,  but  a  dis- 
placed axial  picture  (AP  parallel  to  the  edge  oP\  oo  Poo  or 
oo  P  oo  :  oo  P  oo)  ;  or  simply  an  appearance  of  one  of  the  optic 
axes  according  to  the  degree  of  inclination  of  the  middle  line 
to  the  crystallographic  axes.  Sections  at  right  angles  to  the 
middle  lines  obtain  only  accidentally  and  are  extremely  rare 
(compare  with  the  rhombic  minerals) ;  such,  of  course,  spring 
from  the  zone  oPicoPoo.  In  prismatic  sections  the  displaced 


METHODS  OF  INVESTIGATION. 


37 


axial  picture  or  appearance  of  one  axis  is  not  visible  in  the 
middle  of  the  mineral  leaflet,  but  at  one  side.  If  the  inclina- 
tion of  the  axes  of  elasticity  to  the  crystallographic  axes  is 
very  small,  as,  e.g.,  from  a  :  c  in  mica,  the  mineral  is  apparently 


FIG.  18. — MUSCOVITE.    1  oP. 

Mica  I.  Class. 
(After  Fouque".) 


FIG.  19. — BIOTITE.     ||  oP. 
Mica  II.  Class. 
(After  Fouque".) 


rhombic  (Figs.  18  and  19).  In  the  mica  minerals  the  first  mid- 
dle line  a  differs  but  little  from  the  normals  tooP-A  and  B  are 
the  two  optic  axes  ;  a,  ft,  and  c,  the  axes  of  elasticity. 

If  the  plane  of  the  optic  axes  be  at  right  angles  to  oo  Poo,  an 
appearance  of  one  middle  line  perpendicular  to  oo^Poo  may  al- 
ways be  observed ;  yet  such  an  appearance  is  not  shown  on 
sections  parallel  oP  or  oo  Pec  ;  on  these  a  distorted  axial  pic- 
ture is  again  visible,  AP  parallel  to  the  edge  oP :  ooPoo. 

In  the  Triclinic  Minerals  a  perpendicular  appearance  of  a 
middle  line  obtains  in  none  of  the  pinacoidal  sections,  the  plane 
of  the  optic  axes  is  neither  parallel  nor  at  right  angles  to  a 
pinacoid,  and  only  portions  of  the  interference-figure  can  be 
described  in  the  pinacoidal  sections. 

The  phenomena  of  dispersion  in  monoclinic  and  triclinic 
minerals  cannot  be  established  with  great  precision  by  the  mi- 
croscope or  prove  of  value  in  determining  the  minerals;  in 

general  it  can  only  be  determined  whether  p  ^  v. 

3.  BEHAVIOR  OF  TWINNED  CRYSTALS  IN  POLARIZED  LIGHT. 

Twins  of  the  Regular  System  cannot  be  recognized  as  such 
either  in  parallel  or  convergent  polarized  light,  as  both  indi- 
viduals remain  equally  dark  between  crossed  nicols ;  therefore 


DETERMINATION  OF  ROCK-FORMING  MINERALS. 


According  to  SPco.          According  to  /><» 
FIG.  20. — RUTILE  TWINS. 


the  form  of  the  cross-section  and  the  cleavage  must  be  solely 
regarded  in  the  determination  of  the  law  of  twinning. 

Twins  of  the  Tetragonal  and  Hexagonal  Systems.  —  a.  With 
parallel  axial  systems.    These  also,  for  the  same  reason  as  the 
regular  minerals,  cannot  be  recognized  in  polarized  light. 
b.  On  the  other  hand,  twins  with  inclined  axial  systems  can 

be  recognized  easily  in  parallel 
polarized  light.  In  these  min- 
erals the  chief  axes  and  axes  of 
elasticity  form  an  angle  with  each 
other,  and  the  twinned  crystal  will 
not,  therefore,  act  as  a  unit  in  ex- 
tinguishing the  light  ;  e.g.,  rutile 
C'.C,  =  114°  26'  (Fig.  20).  C,6>re 
the  chief  axes  of  both  individuals,  and  N  is  the  twinning-seam. 
Whdn  one  individual  appears  dark  between  crossed  nicols, 
the  second  becomes  colored.  The  angle  between  the  two 
chief  axes  can  therefore  be  determined,  if  an  edge  of  one  in- 
dividual parallel  to  the  chief  axis  be  first  placed  on  the  centred 
stage  parallel  to  the  nicol  chief  section  so  that  it  is  darkened, 
the  stage  revolved  until  the  second  is  darkened,  and  the  num- 
ber of  degrees  read  through  which  it  was  necessary  to  revolve 
the  stage. 

If  several  individuals  are  twinned  (polysynthetic  twins), 
these  are  wont  to  occur  in  laminations,  as,  e.g.,  in  calcite, 
twinning-plane  —  ^R  (Fig.  21):  in  these, 
in  sections  inclined  to  the  twinning- 
plane,  the  axes  of  elasticity  of  the 
lamellae  i,  3,  5,  etc.,  have  a  similar 
position,  i.e.,  they  extinguish  at  the 
same  instant.  In  sections  parallel  to 

.  ...  ,     . 

the  twinning-plane  no  twinning  stria- 

tions  can  be  observed,  as  in  this  case  but  a  single  individual  is 

met  with. 


FIG.  2i.—  CALCITB  TWIN. 

According  to  -  £/?. 


METHODS  OF  INVESTIGATION. 


39 


If  the  tvvinning-plane  in  calcite  is  the  R-tace,  although 
never  occurring  in  the  rock-forming  individuals,  the  chief  axes 
form  nearly  a  right  angle  with  each  other,  C  :  C^  —  89°  8' ; 
both  individuals  therefore  extinguish  the  ray  at  nearly  the 
same  instant. 

Twins  of  the  Rhombic  System, — The  most  common  examples 
of  this  system  are  : 

1.  Twinning-plane  a  face  of  a  brachydome. 

2.  pyramid. 

3.  "  "  prism. 

In  the  first  two  cases  the  crystallographic  axes  form  an 
angle  with  each  other ;  in  longitudinal  sections  of  such  twins, 
therefore,  no  unit-extinction  between  crossed  nicols  can 
occur.  In  staurolite,  for  example,  the  vertical  axes  c  :  clt 
which  in  this  case  coincide  with  the  axis  of  elasticity  c,  form 
with  each  other  an  angle  of  60°  according  to  the  law  f  Pf ;  but 


FIG.  22.— STAUROLITE  TWINS  ACCORDING  TO  f/'i;  $Po°. 

an  angle  of  90°  according  to  the  law  f  P  oo  ;  i.e.,  in  the  latter 
case  both  individuals  extinguish  together  (Fig.  22). 

A  further  point  of  recognition  for  the  twinning  develop- 
ment in  colored  minerals  lies  in  the  pleochroitic  behavior,  as 
both  individuals,  by  virtue  of  their  different  position  with 
reference  to  the  chief  direction  of  vibration  of  the  polarizer, 
will  be  differently  colored. 


40         DETERMINATION  OF  ROCK-FORMING  MINERALS. 

If  one  of  the  prismatic  faces  is  the  twinning-plane,  a  law 
exemplified,  e.g.,  often  on  aragonite,  rarely  on  cordierite,  etc. 
(Fig.  23),  the  longitudinal  sections  parallel  to  the  vertical  axis, 
when  in  parallel  polarized  light,  show  no  difference  in  the 
direction  of  extinction,  as  the  axes  of  elasticity  of  both  in- 
dividuals coinciding  with  the  ^-axis  are  again  parallel.  The 
two  individuals  in  such  sections,  however,  can  be  accurately 
distinguished  in  convergent  polarized  light,  as  the  same  inter- 
ference-figure does  not  appear  on  both  members ;  but  the  ap- 
pearance of  a  middle  line  on  one  side  and  only  one  of  the 


FIG.  23. — CORDIERITE  TWINS. 
(According  to  v.  Lasaulx.) 

optic  axes  on  the  other,  etc.,  will  be  observed,  the  phenomena 
depending  on  the  direction  of  the  section. 

-Penetration  twins  or  trillings  after  this  law  often  imitate 
the  form  of  an  hexagonal  prism.  Cross-sections  of  such  twins, 
however,  divide  into  six  sectors  in  parallel  polarized  light,  two 
of  which  in  opposite  positions  will  extinguish  at  the  same 
instant.  The  axes  of  elasticity  of  these  three  individuals  are 
inclined  60°  to  each  other ;  an  equal  inclination  of  the  plane 
of  the  optic  axes  in  the  individuals  can  therefore  be  observed 
on  such  twins  by  convergent  polarized  light,  provided  they  are 
not  of  a  mineral  with  the  plane  of  the  optic  axes  parallel  to  oP. 

Twins  of  the  Monoclinic  System. — The  most  commonly-oc- 
curring twinnings  are  according  to  the  law :  twinning-plane 
oo  Poo.  Twinnings  according  to  a  prismatic  face  seldom  occur. 


METHODS  OF  INVESTIGATION. 


bro 


Augite,  amphibole,  epidote,  and  gypsum  may  be 
ward  as  examples  of  the  rock-forming  minerals  with  repeated 
twinning  according  to  ooPoo.  Sections  perpendicular  to  the 
twinning-plane  and  parallel  to  <x>  Poo  will  show,  in  parallel  polar- 
ized light,  in  both  individuals,  an  oblique  extinction  equally 
inclined  to  the  vertical  axis,  i.e.,  the  twinning-seam  or  line 
of  development,  but  in  opposite  directions ;  e.g.,  on  augite 
c  :  c  =  cl  :  c,  =  38°  (Fig.  24).  Such  sections  in  convergent 
light  show  no  difference ;  nor  can  interference-figures  be  recog- 
nized, as  in  these  minerals  the  plane  of  symmetry  is  at  the 


FIG.  24. — AUGITE  TWIN 
according  to  oo/'oo.     ||  oGjP 


FlG.  25.— POLYSYNTHETIC    AUGITE   TWIN 

according  to  oo^oo.     Section  _L  c'-axis. 


same  time  the  plane  of  the  optic  axes.  Such  twins,  with 
parallel  vertical  axes,  can  be  easily  recognized  in  parallel 
polarized  light  as  belonging  to  a  monoclinic  mineral,  as  both 
individuals,  if  rhombic,  would  extinguish  at  the  same  instant. 
As  already  stated,  several  twinning  lamellae  are  often  inter- 
polated according  to  this  law  (Fig.  25) ;  therefore  in  parallel 
polarized  light,  especially  in  sections  at  right  angles  to  the 
vertical  axis,  there  is  often  observed  an  interchange  of  bril- 
liantly-colored lines,  all  parallel  to  a  boundary-line  of  the 
apparently  simple  crystal.  Twins  occur  but  rarely  after  the 
plane  of  a  dome  or  pyramid,  as  in  augite  according  to  —  jP2, 
and  more  rarely  still  penetration-twins  according  to  —  Poo.  In 
the  latter  case  we  are  vigorously  reminded  of  the  staurolite 


DETERMINATION  OF  ROCK-FORMING  MINERALS. 


twinning ;  but  the  extinction  in  these  augite  twins,  so  commonly 
occurring  in  certain  basaltic  rocks,  does  not  occur  parallel  to 
the  vertical  axes  of  both  individuals  on  epidote  (Fig.  26). 
One  or  more  narrow  interpolated  twinning-lamellae  are  often 


120*  34 

r~ 


FIG.  26.— EPIDOTE  TWIN 
according  to  oo^oo.     Section  ||  oojPoo. 


FIG.  27. — TITANITE  TWIN 
according  to  oP.    Section  ||  oojPoo. 


noticed  in  the  hexagonal  sections  parallel  to  the  plane  of  sym- 
metry, which  is  the  same  as  the  plane  of  the  optic  axes  parallel 
to  oo  Pen. 

In  titanite  (Fig.  27),  contact-twins  often  occur  after  the 
law :  twinning-plane  oP.  In  this  case  sections  at  right  angles 
to  the  twinning-plane,  where  they  are  not  parallel  to  oo  Pco, 
also  develop  on  either  side  an  angle  of  extinction  equally  in- 


FIG.  28. — ORTHOCLASE  TWIN 
according  to  the  Carlsbad  and  Baveno  laws. 

clined  to  the  vertical  axis.  Extinction  follows  here  nearly 
parallel  to  the  face  i^oo,  as  the  first  middle  line  is  nearly  per- 
pendicular to  it.  Sections  parallel  oo  Poo  develop  in  convergent 
polarized  light  one  optic  axis  in  each  individual,  but  in  opposite 
directions. 


METHODS  OF  INVESTIGATION. 


43 


The  most  varied  twinning  development  occurs  on  ortho- 
clase  (Fig.  28).  These  will  be  described  more  exactly  in  Part  II. 

Twins  of  the  Triclinic  System. — The  triclinic  rock-forming 
minerals,  especially  plagioclase  and  disthene,  are  quite  com- 
monly polysynthetically  twinned  ;  i.e.,  several  parallel  twin- 
lamellae  are  interpolated  in  the  crystal.  Such  twins  can  be 
recognized  easily  also  in  parallel  polarized  light,  in  that  the 
separate  twinning-lamellae  appear  with  varying  polarization- 
colors,  and  the  directions  of  extinction  do  not  have  the  same 
position  in  two  adjoining  lamellae. 

In  plagioclase  the  "Albite  law"  is  the  most  common: 
twinning-plane  oo  /^oo  (Fig.  29).  Sections  at  right  angles  to  this 
plane  from  the  zone  oP  :  ooPoo  will  always  develop  in  parallel 
polarized  light  the  polysynthetic  twinning-striations.  Such 


FlG.    29.— POLYSYNTHETI_C    PLAGIOCLASE   TWIN.  FlG.  3O.— PLAGIOCLARE   TWINNED 

||  oo/'oo.  according  to  the  A! bite  and  Pericline  laws. 

twins  were  not  possible  in  the  monoclinic  system,  as  the  plane 
in  the  monoclinic  system  ooPoo  corresponding  to  the  plane 
oo  Poo  is  at  the  same  time  a  plane  of  symmetry,  and  such  a 
symmetrical  development  gives  no  twins.  Such  polysynthetic 
twins  are  wanting  in  orthoclase.  It  is  easy,  therefore,  to  dis- 
tinguish orthoclase  from  plagioclase,  although  it  is  not  impos- 
sible for  the  latter  also  to  occur  as  a  simple  twin. 

A  second  less  common  twinning-law  of  plagioclase,  appear- 
ing also  combined  with  the  Albite  law,  is  the  "  Pericline  law :" 
twinning-plane  at  right  angles  to  the  zone  oP  :  oo  Pec,  developed 
after  a  plane  which,  with  the  prismatic  faces,  gives  a  rhombic 
section. 


44          DETERMINATION  OF  ROCK-FORMING  MINERALS. 

The  twinned  developments  of  plagioclase  also  will  be  dis- 
cussed again  at  the  proper  point  in  Part  II.  If  the  Albite  and 
Pericline  laws  are  combined  (Fig.  30),  one  will  observe  in  sec- 
tions cut  approximately  parallel  to  oo/^co  a  double  system  of 
twinning-striations  cutting  each  other  at  nearly  right  angles. 

Disthene   occurs,  though    in   rocks  more  rarely,  as  twins, 
according  to  the  following  laws : 
I.  Twinning-plane  OQ  P  OQ. 

II.  "  at  right  angles  to  the  <r'-axis. 

'  III.  "  "          "          "          £-axis. 

IV.  "  parallel  oP.     This  form  of  twinning  is 

often  repeated  also,  and  is  commonly  observed  in  the  disthene 
occurring  in  rocks. 

And,  finally,  it  may  be  mentioned  that  two  twins  after  one 
definite  law  can  combine  according  to  another  law;  e.g.,  as 
often  occurs  in  plagioclase,  where  two  plagioclase  species 
twinned  after  the  Albite  law  (twinning-plane  OO/^OQ)  are  united 
with  each  other  according  to  the  so-called  Carlsbad  law,  com- 
mon on  orthoclase  (twinning-plane  ooPoo). 

4.  DETERMINATION  OF  THE  INDEX  OF  REFRACTION. 

H.  CLIFTON  SORBY.  On  a  new  method  for  determining  the  index  of  double- 
refraction  in  thin  sections  of  mineral  substances.  Miner.  Mag.  1877, 
No.  6. 

H.  CLIFTON  SORBY.  Determination  of  minerals  in  thin  sections  by  means  of 
their  refractive  indices.  Miner.  Mag.  1878,  No.  8. 

J.  THOULET.  Contributions  a  1'etude  des  proprietes  phys.  et  chim.  des  miner, 
microsc.  Bull.  Soc.  miner..  1880,  III,  62  et  1883,  VI,  184. 

MICHEL  LEVY.     Bull   Soc.  miner.  1883,  VI,  143  et  1884,  VII,  43. 

One  method  of  determining  the  index  of  refraction  in  mi- 
croscopic mineral  particles  has  been  mentioned  before,  under 
the  description  of  the  polarization-microscope  (page  14).  The 
following  method,  applicable  in  many  cases,  was  proposed  first 
by  Thoulet : 

Certain  minerals,  as  olivine,  different  augites,  titanite,  etc., 


METHODS  OF  INVESTIGATION.  45 

show  on  their  sections  a  rough  shagreenous  surface,  considered 
as  almost  characteristic  for  olivine  ;  this  is  much  plainer  if  the 
rock-section  is  not  covered  with  Canada  balsam  and  a  cover- 
ing-glass. This  phenomenon  is  a  result  of  imperfect  polishing 
of  the  slide,  i.e.,  of  the  particular  mineral,  and  disappears  on  a 
more  perfect  finishing.  By  immersion  of  the  mineral  showing 
such  a  rough  upper  surface  in  different  liquids  whose  index 
of  refraction  is  known,  it  is  possible  to  determine  the  index  of 
refraction  of  the  mineral,  as  the  rough  upper  surface  will  dis- 
appear as  soon  as  a  liquid  is  used  whose  index  of  refraction 
equals  or  is  very  near  that  of  the  mineral.  The  liquid  rilling 
the  depressions  in  the  mineral  on  immersion  thus  removes  all 
difference  between  depression  and  elevation  in  the  mineral  leaf- 
lets. Examples  of  such  liquids  where  ;/  =  index  of  refraction 
are: 

Water,  n  —    .34 

Alcohol,  n  —    .36 

Glycerine,  n  =    .41. 

Olive-oil,  n  =    .47 

Beech-oil,  n  =    .50. 

Clove-oil,  n  =    .54 

Cinnamon-oil,          ;/  =    .58. 

Bitter-almond-oil,    n  =     .60. 

Carbon  disulphide,  n  =    .63. 

5.  PLEOCHROISM  OF  DOUBLE-REFRACTING  MINERALS. 
Determination  of  the  Axial  Colors. 

GROTH  and  ROSENBUSCH,  1.  c. 

TSCHERMAK,  Sitzungsber.  d.  k.  Akad.  d.  Wissensch.,  math.-naturw.  Cl.,  Wien. 

1869,  59.  Bd.,  Mai-Heft. 
LASPEYRES.     Groth's  Zeitschr.  f.  Krystallographie.  1880,  IV,  p.  454- 

We  understand  by  pleochroism  that  property  of  double- 
refracting  minerals  whereby  the  light  penetrating  in  different 


46          DETERMINATION  OF  ROCK-FORMING  MINERALS. 

directions  shows  different  colors.  Of  course  only  such  double- 
refracting  minerals  as  are  colored  could  show  the  phenomenon, 
as  it  depends  upon  the  varying  refraction  and  partial  absorption 
of  the  light  penetrating  in  the  different  directions.  Pleochro- 
ism,  or  absorption,  stands  in  closest  relationship  to  double- 
refraction.  Optically-uniaxial  colored  minerals  show  differ- 
ences of  absorption  in  two  directions ;  the  optically-biaxial  in 
three  directions  at  right  angles  to  each  other  and  corresponding 
to  the  different  axes  of  elasticity. 

Optically-TJniaxial  Minerals. — On  looking  through  such  a 
mineral  in  a  direction  at  first  parallel  and  then  perpendicular 
to  the  chief  axis,  a  difference  in  color  will  be  noted.  The  color 
observed  on  looking  through  parallel  to  the  chief  axis  is  called 
the  "  basal  color"  (Basis-far be),  the  color  at  right  angles  to  it 
the  "  axial  color"  (A.ren-farbe).  If  a  transverse  section  of  such 
a  mineral  be  examined  in  a  polarization-microscope,  with  only 
the  polarizer  in  position, — the  single  nicol  thus  performing  the 
duty  of  a  dichroscope, — and  the  section  be  turned  through  one 
complete  revolution,  no  difference  in  color  is  noticed,  as  only 
rays  vibrating  at  right  angles  to  the  optic  axis  are  in  the  field, 
and  there  is  no  double-refraction  in  the  direction  parallel  to 
the  chief  axis.  If,  on  the  other  hand,  a  longitudinal  section  be 
placed  in  the  microscope,  a  change  of  color  is  noticed  on  re- 
volving the  stage.  The  greatest  difference  in  absorption  is 
noticed  first  when  the  chief  axis  is  parallel  to  the  nicol  chief 
section,  and  secondly  when  it  is  at  right  angles  to  it.  Thus, 
e.g.,  tourmaline  (Fig.  31;  xy  denoting  the  optical  chief  section 
of  the  polarizer,  c  the  chief  axis,  a  and  c  the  two  axes  of  elas- 
ticity), which,  as  is  well  known,  absorbs  the  ordinary  ray  much 
more  powerfully  than  the  extraordinary,  appears  nearly  black 
when  its  <:-axis  is  at  right  angles  to  the  shorter  diagonal  of  the 
polarizer,  but  shows  a  light  gray  or  blue  color  when  the  r-axis 
is  parallel  to  the  nicol  chief  section.  Consequently  the  or- 
dinary ray  (o)  vibrating  at  right  angles  to  the  chief  axis  is 


METHODS  OF  INVESTIGATION. 


47 


transmitted  with  darker  colors;  the  extraordinary  ray  (f)  vi- 
brating parallel  to  the  chief  axis  with  a  lighter  gray  or  bluer 
color.  As  the  double-refraction  of  tourmaline  is  negative, 
therefore  a  =  c  and  c  _L  c.  We  can  thus  express  the  axial 
colors :  a  ==  light  gray  (e),  c  =  black  (o).  And  in  general,  in 
minerals  with  negative  double-refraction  the  ordinary  ray  is 
more  strongly  absorbed;  in  those  with  positive  double-refraction, 


FIG.  31. — DICHROISM.     (Tourmaline.    Section  |i  r-axis.) 
(After  Fouque\) 


the  extraordinary  ray.  The  color  with  which  o  is  transmitted 
corresponds  with  the  basal  color,  while  the  axial  color  is  com- 
pounded from  both  of  the  colors  for  o  and  e.  In  order  to 
observe  the  colors  of  the  faces,  the  under  nicol,  the  polarizer, 
is  of  course  also  removed,  and  the  investigation  carried  on  by 
ordinary  light. 

Optically-Biaxial  Minerals. — The  differences  of  absorption  are 
developed  in  optically-biaxial  minerals  in  three  directions  at 
right  angles  to  each  other  and  coinciding  with  the  three  axes 
of  elasticity  for  the  most  part.  We  discriminate  here,  there- 
fore, between  three  face-colors  and  three  axial  colors.  The 
three  axial  colors  corresponding  to  the  axes  of  elasticity  are 
designated  by  a,  b,  and  c  ;  each  face-color  is  composed  of  two 
axial  colors.  If  one  looks  through  a  cordierite  crystal,  e.g., 
through  the  plane  oP,  i.e.,  in  the  direction  of  the  vertical  axis, 
here  coinciding  with  the  axis  of  elasticity  a,  it  appears  blue  ; 
that  is  to  say,  the  face-color  A  which  is  composed  of  the  axial 


48 


DETERMINATION  OF  ROCK-FORMING  MINERALS. 


colors  b  and  C  is  blue;  parallel  co/^co  (c)  the  face-color  C  is 
yellowish  white  from  the  composition  of  a  and  b ;  and  parallel 
to  oo/^oo  (b)  the  face-color^  is  a  bluish  white  from  composition 
of  the  axial  colors  a  and  c.  On  the  other  hand,  the  axial  colors 
for  cordierite  are:  a,  yellowish  white;  b,  light  Prussian  blue:  c, 
dark  Prussian  blue.  The  determination  is  effected  in  the  fol- 
lowing manner  :  If  a  cross-section  of  a  crystal  whose  optical 
orientation  is  known  be  selected,  e.g.,  a  section  of  hypersthene 
(Fig.  32)  at  right  angles  to  the  ^'-axis  (parallel  oP\  the  axial  colors 
b  and  c  can  be  determined  in  it  with  the  polarizer ;  on  turning 
the  stage,  first  the  brachy-axis  (a  =  a)  and  then  the  macro-axis 


FIG.  32.— TRICHROISM.    (Hypersthene.    Sections  J-c'-axis  and  i|  «>A».) 

(b  =  b)  is  parallel  to  the  nicol  chief  section,  and  above  the 
polarizer.  Another  cross-section  of  mineral  is  needed  in  order 
to  determine  the  axial  color  for  c.  This  section  in  the  case 
cited  can  be  either  parallel  ooPco  or  oo/^co.  Parallel  to  oo/>oo 
two  axial  colors,  a  and  c,  can  again  be  determined ;  the  axial 
color  c  is  observed  so  soon  as  the  vertical  axis  (cr  =  c)  coincides 
with  a  nicol  chief  section,  that  for  a  so  soon  as  the  brachy-axis 
coincides.  The  axial  color  a,  therefore,  was  determined  twice, 
and  must  correspond  in  both  cases  if  the  sections  were  of  equal 
thickness. 

If  pleochroitic  minerals  are  examined  in  extremely  thin 
sections,  as  is  always  the  case  in  rock  thin  sections,  the  differ- 
ences of  absorption  are  often  imperceptible.  This  is  true  of 
cordierite  and  andalusite,  while  tourmaline,  e.g.,  shows  the 


METHODS  OF  INVESTIGATION.  49 

most  marked  dichroism  even  in  the  thinnest  needles.  For  this 
reason  it  is  advisable  to  prepare  a  somewhat  thicker  section 
from  the  rock  under  examination  for  the  investigation  of  the 
optical  properties  of  the  larger  mineral  constituents. 

The  power  of  absorption  in  different  directions  in  any  mineral 
is  represented  by  an  >  or  <  annexed  to  the  axes  of  elasticity ; 
in  tourmaline,  e.g.,  o  >  f  or  c  >  a,  i.e.,  the  ordinary  ray  is  more 
powerfully  absorbed  than  the  extraordinary.  In  cordierite 
C  >  b  >  a,  or,  as  the  axes  of  elasticity  in  the  rhombic  minerals 
coincide  with  the  crystallographic  axes,  in  these  according  to 
the  optical  orientation  corresponding  b  >  a  >  c  ;  i.e.,  the  absorp- 
tion in  cordierite  is  greatest  in  the  direction  of  the  macro-axis. 

In  tetragonal  and  hexagonal  minerals  the  directions  in  which 
the  greatest  color-difference  can  be  recognized — Laspeyres 
calls  them  "  axes  of  absorption" — coincide  with  the  two  axes 
of  elasticity,  i.e.,  are  parallel  and  at  right  angles  to  the  chief 
axis ;  in  the  rhombic,  with  the  three  axes  of  elasticity,  i.e.,  the 
crystallographic  axes ;  in  the  monoclinic  and  triclinic  minerals, 
however,  according  to  the  latest  investigations  of  Laspeyres, 
the  three  axes  of  absorption  do  not  coincide  with  axes  of  elas- 
ticity, but  yet  are  at  right  angles  to  each  other. 

In  the  monoclinic  minerals  there  appears  to  be  but  one 
coincidence  of  an  absorption-axis,  and  that  with  the  ortho- 
diagonal  ;  while  each  of  the  others,  lying  in  the  plane  of  sym- 
metry, forms  an  angle  with  the  axes  of  elasticity.  Colorless 
double-refracting  minerals,  e.g.,  apatite,  often  show  pleochroism 
as  a  consequence  of  the  regular  inclosures  of  colored  particles 
or  other  mineral  fragments. 

And,  finally,  it  may  be  mentioned  that  the  axial  colors  of 
pleochroitic  minerals  do  not  remain  constant,  in  that  often  on 
cross-sections  of  one  and  the  same  mineral  now  C  >  a  >  b  and 
now  c  >  b  >  a,  and  so  on,  are  observed  ;  or  one  and  the  same 
mineral  can  be  now  feebly  and  now  powerfully  pleochroitic. 
Nevertheless  pleochroism  is  a  characteristic  for  certain  min- 


5O          DETERMINATION  OF  ROCK-FORMING  MINERALS. 

erals,  as  andalusite,  cordierite,  tourmaline,  hypersthene,  horn- 
blende, biotite,  and  others,  and  thus  lends  its  aid  to  their 
determination. 


B.  Chemical  Methods  of  Investigation. 

The  chemical  examination  of  rocks  should  go  hand  in 
hand  with  the  microscopical  investigation ;  a  quantitative 
analysis  will  always  give  a  welcome  explanation  of  the  min- 
eralogical  composition,  or  at  least  will  confirm  the  microscopi- 
cal examination  to  a  greater  or  lesser  degree.  It  is,  however, 
impossible  to  determine  the  component  minerals  or  to  give 
their  individual  chemical  composition  from  such  a  rock-analy- 
sis alone.  In  order  that  the  rock-forming  minerals  may  be 
separately  analyzed  and  their  chemical  composition  correctly 
determined  it  is  necessary  to  separate  them  from  each  other. 
Such  a  mechanical  separation  can  be  simply  effected  with  a 
needle  beneath  a  microscope,  when  only  a  small  fragment  may 
be  needed  for  a  qualitative  chemical  test  of  the  minerals  ;  or  it 
may  be  effected  by  solutions  of  high  specific  gravity,  thus  tak- 
ing advantage  of  the  differing  specific  gravities  of  the  minerals 
for  obtaining  larger  quantities  of  mineral  for  the  quantitative 
chemical  investigation.  There  is  also  another  advantage  in  this 
latter  method,  as  the  specific  gravities  of  the  separate  mineral 
components  are  thus  known. 

If  the  rock  under  examination  is  coarsely  granular,  the  sep- 
arate components  often  can  be  distinguished  with  the  naked 
eye  and  the  different  cleavage-leaflets  be  examined  optically 
as  well  as  by  chemical  qualitative  and  quantitative  analysis. 
In  this  separation  it  is,  e.g.,  impossible  to  separate  several  feld- 
spars from  each  other  in  case  they  occur  in  the  same  rock.  Such 
a  separation  of  the  components  is  also  impossible  in  the  fine- 
grained rocks.  In  order  to  examine  chemically  the  rock-con- 
stituents in  such  cases  and  thus  obtain  a  clue  in  the  determina- 


METHODS  OF  INVESTIGATION.  51 

tion,  the  microchemical  reactions  are  applied  ;  the  component 
under  examination  beneath  the  microscope  is  dissolved  either 
directly  on  the  rock-section  or  on  detached  granules,  and 
treated  with  such  reagents  as  give  exceptionally  characteristic 
precipitates.  Sometimes  a  more  exact  and  careful  mechanical 
separation  of  the  mineral  components  is  attempted  by  treat- 
ing the  powdered  rocks  with  solutions  of  high  specific  gravity. 
A  partial  analysis  of  the  portion  of  rock  soluble  or  insoluble 
in  hydrochloric  acid  in  many  cases  gives  valuable  conclusions 
and  simplifies  the  determination  of  the  constituents. 

MICROCHEMICAL  METHODS. 

H.  ROSENBUSCH,  1.  c.,  p.  loy,  und  N.  Jahrb.  fiir  Min.  und  Geol.  1871,  p.  914. 

F.  ZIRKEL.     Basaltgesteine  und  Lehrb.  d.  Petrographie. 

A.   STRENG.     Ueber  die   mikroskopische    Unterscheidung  von    Nephelin   und 

Apatit.     Tschermak's  Miner.  Mitth.  1876,  p.  167. 
E.  BOKICKY.     Elemente  einer  neuen  chemisch-mikroskopischen  Mineral-  und 

Gesteinsanalyse.      Archiv    d.    naturw.     Landesdurchforsch.     Bohmens. 

III.  Bd.,  V.  Abthlg.,  Prag,  1877. 
SZABO.     Ueber  eine   neue    Methode,    die    Feldspathe   auch    in    Gesteinen   zu 

bestimmen.     Budapest,  1876. 
TH.  H.  BEHRENS.     Mikrochemische  Methoden  zur  Mineralanalyse.    Verslagen 

en   Mededeelingen  der  k.   Academic  v.  Wetenschappen.     Amsterdam, 

1881. — Afdeeling  Natuurkunde.     2.  Reeks,  XVII.  Deel.  p.  27 — 73. 
A.  STRENG.     XXII.  Ber.  der  oberhesisschen  Ges.  f.  Nat.  u.  Heilkunde.     1883, 

p.  258  u.  260. 

E.  BORICKY.     N.  Jahrb.  f.  Min.  u.  Geol.  1879.  P-  564- 
MICHEL  LEVY  et  L.  BOURGEOIS.    Compt.  rendus  1882,  20  mars,  and  Bull.  Soc. 

miner.  1882,  V.  p.  136  (Reaction  auf  Zirkonerde). 
SCHONN.     Zeitschr.  fiir  analyt.  Chemie.  1870.   IX.  p.  41  (Reaction  auf  Titan- 

saure). 
A.  KNOP.     N.  Jahr.  f.  Min.  u.  Geol.  1875,  p.  74. 

Hydrochloric  acid  has  been  applied  for  a  long  period  as  a 
microscopical  reagent  in  investigations  of  rocks.  Zirkel  (comp. 
Petrogaphie,  II.  p.  293,  1870)  applied  it  most  advantageously 
in  discriminating  between  the  varieties  of  plagioclase  allied  to 
anorthite  and  those  related  to  albite,  and  between  magnetite 


52          DETERMINATION  OF  ROCK-FORMING  MINERALS. 

and  ilmenite.  The  application  of  hydrochloric  acid  for  the 
determination  of  calcite  in  rocks  has  been  known  for  a  much 
longer  period ;  also  in  the  recognition  of  silicates  soluble  in 
this  acid,  as  nepheline,  members  of  the  meionite  group,  etc. 
Thus  Roth  (1865)  rightly  conjectured  the  presence  of  melilite 
in  the  basaltic  lavas  from  Eifel  because  of  the  large  amount 
of  calcium  dissolved  in  the  acid. 

In  such  a  testing  of  the  rock-constituents  regard  is  had  first 
of  all  for  the  solubility,  and  secondly  for  the  products  of  the 
decomposition  effected  by  the  acid ;  as  the  evolution  of  CO2 
from  calcite,  the  deposition  of  the  NaCl-cubes  on  evaporating 
a  drop  of  the  test  for  nepheline,  the  appearance  of  the  gelatin- 
ous SiO2  on  treating  olivine  with  hydrochloric  acid,  etc. 

•  In  such  examinations  the  testing  is  made  with  powdered 
rock  by  examining  microscopically  the  rock-section  or  powder 
both  before  treatment  with  acid  and  also  afterward  if  a  residue 
remains.  In  the  second  case  the  testing  is  undertaken  directly 
on  the  slide  without  a  glass  cover.  There  are  great  evils  in 
either  case  ;  in  the  one  in  that  it  is  difficult  to  recognize  the 
minerals  in  powdered  condition  and  thus  determine  what  has 
been  dissolved  away,  and  in  the  other  in  that  in  treating  the 
section  with  acids  the  whole  section  crumbles  away  and  is 
destroyed. 

A.  Streng  has  recommended  a  method  of  isolating  the 
minerals  of  a  thin  section  for  microchemical  study  which  is  to 
be  recommended  in  many  cases.  If  a  mineral  granule  in  a  thin 
section  is  treated  with  acid,  it  is  almost  always  unavoidable 
that  the  drop  of  solvent  may  touch  also  the  other  neighboring 
particles,  react  on  them,  and  thus  render  the  chemical  reactions 
questionable.  This  evil  can  be  remedied  by  first  covering  the 
section  with  a  perforated  covering-glass  which  is  coated  on  the 
under  side  with  fluid  boiled  Canada  balsam,  so  that  the  open- 
ing of  about  -J— I  millimetre  in  diameter  is  opposite  the  mineral 
particle  to  be  tested.  The  Canada  balsam  filling  the  opening 


METHODS  OF  INVESTIGATION.  53 

may  be  easily  removed  by  alcohol.  Such  perforated  covering- 
glasses  can  be  easily  prepared  by  treatment  with  hydrofluoric 
acid.  An  ordinary  covering-glass  is  first  dipped  in  melted  wax 
and  allowed  to  cool;  a  hole  ^-i  mm.  diameter  is  then  made 
through  the  wax,  and  concentrated  hydrofluoric  acid  dropped 
on  the  bared  opening  until  a  hole  is  eaten  through  the  glass  at 
this  point.  The  wax  is  then  removed  from  the  covering-glass. 

The  reaction  for  distinguishing  between  nepheline  and 
apatite,  first  proposed  by  Streng  (1876),  deserves  special  men- 
tion as  one  nearly  always  accomplishing  its  purpose.  Both 
minerals  occur  in  rocks  very  commonly,  and  are  remarkably 
similar — hexagonal  (ooP.oP.P),  optically  negative,  and  color- 
less. 

The  microchemical  reactions  for  apatite  are : 

(a)  Reaction  for  phosphoric  acid.     A  drop  of  concentrated 
nitric-acid  solution  of  ammonium  molybdate  is  transferred  with 
a  glass  rod  to  the  apatite  crystal  lying  exposed,  i.e.,  not  covered 
by  other  minerals  of  the  section  ;  the  whole  of  the  thin  section 
within  the  field  of  the  microscope  not  protected  by  glass  is 
thus  covered.     A  muscovite  or  glass  leaflet  is  often  cemented 
with  glycerine  to  the  objective  to  protect  the   lens,  which  in 
such  experiments  is  easily  attacked  by  the  acid  vapors.     The 
apatite  dissolves  slowly  in  the  nitric  acid  of  the  reagent,  form- 
ing beautiful  yellow  grains  and  small  octahedra  of  the  ammo- 
nium  phospho-molybdate   (ioMoO3  +  PO4(NH4)3  +  iJH2O). 
These  yellow  crystals  are  wreathed  about  the  apatite  and  not  in 
the  former  position  of  the  apatite  crystal,  as  here  the  excess  of 
phosphoric  acid  prevents  the  formation  of  a  precipitate. 

(b)  Reaction    for   lime.     A  crystal  of   apatite  in  the   thin 
section  is  dissolved  in  hydrochloric  or  nitric  acid  and  a  drop 
of  sulphuric  acid  added :    fine   white    feathery  aggregates   of 
gypsum  are  formed  round  about  the  point  previously  occupied 
by  the  apatite.    If  a  crystal  of  apatite  is  treated  with  sulphuric 
acid  alone  it  is  not  dissolved,  as  a  thin  coating  of  gypsum  is 


54         DETERMINATION  OF  ROCK-FORMING  MINERALS. 

formed  which  prevents  the  further  action  of  the  acid  on  the 
'•apatite. 

The  reaction  of  Streng  for  phosphoric  acid  is  the  surest  and 
most  exact  if  it  is  carried  out,  not  on  the  thin  section  directly, 
but  on  an  isolated  granule  :  or  if  the  thin  section  be  treated 
with  dilute  nitric  acid,  the  solution  taken  up  with  a  capillary 
pipette,  evaporated,  again  dissolved  in  dilute  nitric  acid,  and 
the  reaction  completed  on  an  ordinary  object-glass. 

Nepheline  can  be  recognized  from  the  negative  results  to 
the  reactions  given  above  for  apatite,  as  well  as  by  a  reaction 
with  hydrochloric  acid  ;  if  a  drop  of  the  acid  be  deposited  on 
a  crystal  under  examination  it  is  easily  decomposed,  i.e.,  dis- 
solved. After  some  time  numbers  of  minute  colorless  cubes 
of  sodium  chloride,  easily  recognized,  are  formed  in  the  cavity 
formerly  occupied  by  the  crystal.  They  are  formed  by  the 
action  of  the  hydrochloric  acid  on  sodium  silicate,  and  are 
difficultly  soluble  in  the  concentrated  acid. 

A.  Streng  has  recently  found  acetate  of  uranium  to  be  an 
excellent  reagent  for  sodium.  If  a  drop  of  concentrated  solu- 
tion of  acetate  of  uranium  be  added  to  the  residue  from  the 
solution  of  a  silicate  in  hydrochloric  acid,  clearly  defined,  bright 
'O  O  O 


yellow  tetrahedra  I —  .  -     -  or  —  .  oo  O )  of    sodium    uranate, 

\2  2  2  / 

difficultly  soluble  in  water,  are  formed.  More  rarely  penetra- 
tion-twins after  a  tetrahedral  face  occur,  and  in  polarized  light 
can  be  easily  distinguished  from  the  double-refracting,  rhom- 
bic, nearly  cubical  crystals  of  the  acetate  of  uranium. 

A.  Knop  has  recommended  a  reaction  for  the  recognition 
of  members  of  the  hauyn  group,  which  when  colorless  are 
difficultly  distinguishable  from  apatite  or  nepheline  sections. 
The  thin  section  of  the  rock  bearing  the  hauyn  is  carefully 
loosened  from  the  object-glass  by  warming,  and  is  washed  clean 
with  alcohol.  The  clean  section  is  introduced  into  a  platinum 
crucible,  and  as  much  flowers  of  sulphur  as  can  be  taken  up  on 


METHODS  OF  INVESTIGATION.  CC 

-,  ^^W  »  «J 


the  point  of  a  knife  added.  If  now  the  en 
glowing  for  some  minutes,  whereby  the  sulplrl 
fills  the  crucible,  and  then,  still  covered,  is  allowed 
ferrous  compounds  appear  blackened,  while  the  hauyn  is  con- 
spicuous among  the  rock-components  by  the  beautiful  azure- 
blue  color.  The  other  rock-forming  minerals  do  not  become 
blue  on  heating  in  sulphur-vapor.  Knop  does  not  state,  how- 
ever, whether  sodalite,  like  hauyn,  becomes  blue. 

These  few  characteristic  microreactions  have  reference, 
however,  to  an  extremely  limited  number  of  the  rock-forming 
minerals — nepheline,  apatite,  and  hauyn.  The  necessity  for  a 
method  of  complete  microchemical  qualitative  analysis  of 
the  rock-constituents  has  been  remedied  by  Boricky  and 
Behrens. 

Boricky  s  Microchemical  Method. 

Chemically  pure  hydrofluosilicic  acid  is  the  only  reagent 
required.  It  should  contain  13  per  cent  acid,  and  must  be 
absolutely  pure  ;  i.e.,  when  allowed  to  dry  on  a  layer  of  balsam 
on  an  object-glass  it  must  leave  no  residue  of  silico-fluoride 
crystals.  It  cannot,  therefore,  be  prepared  or  stored  in  glass 
bottles.  Almos?  all  of  the  rock-fprming  minerals  are  attacked 
more  or  less  by  strong  hydrofluosilicic  acid.  It  is  therefore 
available  for  the  formation  of  the  silico-fluorides,  which  dis- 
solve in  the  solution  of  hydrofluosilicic  acid,  and  after  evapo- 
ration of  this  solution  appear  as  beautifully-developed  crystals, 
characteristic  for  the  different  elements  or  groups  of  elements. 

The  microchemical  tests  with  this  acid  can  be  carried  out 
either  directly  on  the  rock-section  without  a  glass  cover,  or, 
better  yet,  on  minute  particles  of  the  minerals  of  about  the  size 
of  a  pin's  head,  on  an  object-glass  coated  with  Canada  balsam. 
One  or  two  drops  of  the  hydrofluosilicic  acid  are  transferred 
with  a  caoutchouc  rod  to  the  mineral  granule  under  examina- 


56         DETERMINATION  OF  ROCK-FORMING  MINERALS. 

tion,  and  the  preparation  is  allowed  to  rest  quietly  in  a  place 
free  from  dust,  preferably  at  a  temperature  of  about  18°  C., 
until  the  drop  has  dried  away. 

If  the  mineral  is  easily  attacked  by  the  acid,  all  of  the 
metals  are  generally  found  after  evaporating  the  solution  in 
their  several  peculiar  crystalline  forms,  and  in  about  the  same 
proportion  as  in  the  mineral.  If  the  mineral  is  but  slightly 
attacked,  only  those  metals  most  easily  soluble  can  be  proven, 
and  the  same  mineral  fragment  must  be  treated  again  with  the 
acid ;  in  the  latter  case  it  is  often  of  advantage  to  treat,  in  a 
small  platinum  dish,  first  with  hydrofluoric  acid  and  then  with 
hydrofluosilicic  acid,  evaporate  to  dryness,  redissolve  in  water, 
and  allow  a  drop  to  evaporate  on  an  object-glass. 

Thin  sections  are  more  easily  attacked  than  granules  or 
cleavage-pieces,  and  must  be  exceedingly  thin.  It  is  better  if 
the  test  is  taken  from  carefully-selected  mineral  particles,  as 
sections  become  coated  with  a  dull  white  crust.  The  silico- 
fluorides  crystallize  most  perfectly  when  lixiviated  with  boiling 
water,  and  the  solution  allowed  to  cool  on  another  object- 
glass.  The  silico-fluorides  are  always  in  minute  crystals,  and 
are  best  observed  under  200-400  diameters.  They  are  distin- 
guished by  their  crystalline  forms,  and  there  appears : 

1.  Potassium  Silico-fluoride  in  skeleton  groups  of  small  crys- 
tals of  the  regular  system  clearly  defined,  generally  oo  O  oo,  also 
often  with  O  and    oo  O.     Yet  potassium   silico-fluoride   often 
crystallizes  in  larger,  apparently  rhombic  crystals  of  the  form 
ooP;/  .  raPoo,  if  the   acid  was   in    excess   or   the  evaporation 
occurred  at  lower  temperatures  (12°  C.),  or  in  presence  of  a 
large  amount  of  sodium. 

2.  Sodium  Silico-fluoride  (Fig.  33)  in  short  hexagonal  columns 
with  oP.  P,    also  oo  P2  ;  imperfect   crystals   are   barrel-shaped. 
The  more  calcium  silico-fluoride  present  the  larger  the  crystals. 
Easily  soluble  in  water. 

3.  Calcium  Silico-fluoride  (Fig.  34)  in  peculiar,  long,  pointed, 


METHODS  OF  INVESTIGATION. 


57 


spindle-shaped  crystals,  often  grouped  in  rosettes;  the  combina- 
tion of  parallel  straight  lines  and  planes  is  characteristic  for 


FIG.  77. — SODIUM  SILICO-FLUORIDE. 


33 


(After  Boricky.) 


FIG.  34. — CALCIUM  SILICO-FLUORIDE. 
(After  Boricky.) 


this  compound.     It  crystallizes  in   monoclinic  crystals,  and  is 
easily  soluble  in  water. 

4.  Magnesium  Silico-fluoride  (Fig.  35)  appears  in  rhombohe- 
dra  with  polar  edges  truncated  by 

oR  and  combinations  of  R  .  oo/^  or 
R .  oo  Pa .  oR ;  all  of  the  crystals  have 
well-defined  edges  and  faces.  It 
often  appears  also  in  rhombohedra 
elongated  in  one  direction,  or  in 
cruciform,  hook-shaped,  or  feathery 
figures.  It  is  easily  soluble  in  water. 

5.  Iron  Silico-fluoride    cannot    be 
distinguished    from   magnesium  sili- 
co-fluoride;    the    same  with  manga- 
nese silico-fluoride  ;  while  strontium 

silico-fluoride  can  scarcely  be  distinguished  from  calcium  silico- 
fluoride. 

Lithium  Silico-fluoride  appears  generally  in  regular  flat  hexag- 
onal pyramids,  where  one  pair  of  faces  is  sometimes  remarkably 


FIG.  35. — MAGNESIUM  SILICO- 
FLUORIDE. 
(After  Boricky.) 


58          DETERMINATION  OF  ROCK-FORMING  MINERALS. 

developed  ;  barium  silico-fluoride  in  extremely  minute,  short, 
pointed  needles. 

Distinction  between  the  Silico-fluorides  of  Calcium  and  Stron- 
tium.— If  a  drop  of  sulphuric  acid  diluted  with  an  equal  bulk 
of  water  is  added  to  the  silico-fluorides,  the  crystals  of  calcium 
are  immediately  surrounded  with  a  thick  fringe  of  mono- 
clinic  gypsum  crystals,  while  those  of  strontium  change  but 
slowly. 

Distinction  between  the  Silico-fluorides  of  Iron,  Manganese, 
and  Magnesia. — These  can  be  distinguished  either  by  subject- 
ing to  the  action  of  chlorine  gas  for  about  twenty  minutes, 
when  the  magnesium  silico-fluoride  remains  colorless,  the  iron 
becomes  yellow  and  the  manganese  red  ;  or  these  silico- 
fluorides  can  be  distinguished  by  the  reaction  with  ammonium 
sulphide,  when  the  silico-fluoride  of  magnesium  remains  color- 
less, while  the  iron  is  blackened  and  the  manganese  becomes 
reddish-gray  and  granular. 

The  fluorides  of  Fe,  Mn,  Co,  Ni,  and  Cu  can  be  distinguished 
also  by  their  reaction,  with  potassium  ferrocyanide.  If  this 
solution  be  dropped  on  the  silico-fluorides  the  corresponding 
ferrocyanides  will  be  formed,  which  can  be  recognized  from 
the  characteristic  color :  Fe  is  blue,  Mn  brown,  Cu  red,  Co  dark 
green,  and  Ni  light  green. 

This  method  has  many  disadvantages ;  e.g.,  it  is  impossible 
to  prove  by  it  the  presence  of  alumina ;  the  distinction  be- 
tween the  silico-fluorides  of  iron  and  magnesium  is  difficult 
and  detailed ;  the  calcium  silico-fluoride  crystals  are  also  in- 
sufficiently characteristic.  Nevertheless  it  is  advantageously 
employed,  especially  in  testing  for  the  alkalies. 

Th.  A.  Behrens  has  proposed  another  complete  system  of 
microchemical  methods  for  use  in  petrography.  In  this 
method  also  a  series  of  new  and  admirable  microreactions  are 
introduced.  If  a  combination  of  these  two  methods — that  of 
Boricky  for  the  determination  of  the  alkalies,  and  of  Behrens 


METHODS  OF  INVESTIGATION.  59 

— be  effected,  a  complete  qualitative  analysis  in  many  cases 
can  be  carried  out  with  the  microscope.  In  this  latter  method, 
however,  the  operation  cannot  be  carried  out  on  the  rock- 
section  itself. 


Behrenss  Micrcchemical  Method. 

Preparation  of  the  Mineral. — The  minerals  to  be  examined 
must  always  be  separated  from  the  mass  of  the  rock.  In  the 
coarse-grained  rocks  this  is  easily  done  by  picking  out  the 
pieces  from  the  coarse  rock-powder  either  under  the  micro- 
scope or  with  a  pocket-lens.  In  the  fine-grained  rocks,  where 
the  rock-constituents  can  no  longer  be  distinguished  in  the 
powder,  the  mineral  particle  is  removed  from  the  slide  by 
aid  of  the  microscope  and  a  lance-shaped  needle ;  the  section 
is  ground  until  the  desired  mineral  granule  is  transparent 
and  partly  polished.  The  isolation  of  the  desired  mineral  is 
effected  by  gradually  breaking  away  the  section  from  the  edge. 
The  isolation  of  the  mineral  is  lightened  if  the  object-glass  is 
first  warmed,  and  the  Canada  balsam  under  the  rock-leaflet 
thus  softened.  The  isolated  mineral  particle,  of  at  least  0.3 
mm.  diameter  and  o.i  mg.  in  weight,  is  cleaned  and  pulverized 
in  an  agate  mortar  beneath  a  piece  of  filter-paper  to  prevent 
loss. 

The  Testing. — The  tests  are  made  in  a  hemispherical  plati- 
num dish  about  I  cm.  in  diameter,  closed  by  a  concave  platinum 
cover  ;  the  reagent  employed  is  chemically  pure  hydrofluoric 
acid,  or  ammonium  fluoride,  or  concentrated  hydrochloric  acid. 
Two  or  three  drops  of  either  acid  are  transferred  to  the  small 
dish,  and  the  mineral,  finely  powdered,  added.  The  mixture 
is  heated,  and,  if  necessary,  hydrofluoric  acid  added  a  second 
time,  and  the  evaporation  repeated.  The  dried  fluorides  are 
then  evaporated  with  concentrated  sulphuric  acid  until  volu- 


60         DETERMINATION  OF  ROCK-FORMING  MINERALS. 

minous  clouds  of  the  gray  acid-vapors  appear.  The  sulphuric 
acid,  however,  must  not  be  completely  volatilized  ;  it  is  advis- 
able, therefore,  to  repeat  the  evaporation  with  a  drop  of  sul- 
phuric acid.  The  decomposed  mass  is  then  dissolved  in  water, 
the  platinum  capsule  being  about  half  rilled,  and  the  contents 
evaporated  by  gentle  heat  until  each  centigram  of  solution 
contains  about  o.i  mg.  substance. 

A  drop  of  this  solution  is  transferred  by  a  capillary  pipette 
to  a  slide  without  a  covering-glass  to  facilitate  evaporation, 
and  is  placed  beneath  the  microscope.  Two  hundred  diameters 
is  the  best  magnifying  power.  The  objective  here  also 
must  be  protected  by  a  leaf  of  muscovite  cemented  with 
glycerine. 

This  drop  is  examined  first  for 

Calcium. — If  the  mineral  was  calciferous,  free  crystals  of 
gypsum  (Fig.  36)  separate  on  evaporation ;  the  columns  are 

thin,  of  oo  P.  oo  Pco  .  P,  generally  lying 
on  coPco  or  arranged  in  rosettes. 
Often  larger  crystals  of  the  well- 
known  swallow-tail  twins  are  discern- 
ible in  the  outer  edge  of  the  drop. 
The  presence  of  0.0005  mg-  CaO 
can  be  demonstrated  by  this  reaction. 
If  a  smaller  amount  of  lime  is  present, 
or  the  gypsum  separates  too  slowly, 
the  slide  with  the  drop  is  moistened 

FIG.  36.— GYPSUM.  111          r^i 

(After  Behrens.)  with  alcohol.  The  crystals  then 

formed  are,  however,  smaller  and  less  distinct,  but  the  sensi- 
tiveness of  the  reaction  is  quadrupled. 

The  same  drop  is  searched  for 

Potassium. — A  drop  of  concentrated  platinum  chloride  is 
added  by  means  of  a  platinum  wire  to  the  drop  to  be  tested. 
Crystals  of  the  double  chloride  of  platinum  and  potassium 
(Fig.  37,  a)  are  formed  within  a  few  minutes,  and  generally  on 


METHODS  OF  INVESTIGATION.  6l 

the  outer  edge  of  the  drop.     They  are  sharply-defined   octa- 

hedra  of  high  refractive  power  and  of  a  bright  yellow  color. 

If  a  concentrated  solution  was  employed,  clover-leaved  trillings 

and    fourlings    also    appear.     The 

crystals  are    formed   more    rapidly 

in  chloride  solution  than  in  sulphate 

solution,  and  are  smaller.     A  large 

excess  of   sulphuric  acid  prevents 

their   formation.     0.0006   mg.  K2O 

can   be   demonstrated    by  this   re- 

action. 

Sodium  is   proved  with   cerium 
sulphate.     A  drop  of  the  concen- 

trated solution  of  this  reagent,  and      a.  POTASSIUM-PLATINUM  CHLORIDE. 
,  ,  r      i  1       •  r  b.  POTASSIUM  FLUOBORATE. 

another  drop  of  the  solution  from  (After  Be 


the  decomposed  mineral,  are  placed  on  a  slide  about  5  mm. 
apart,  and  joined  by  a  thread  of  glass.  Tufts  of  cerium 
sulphate  appear  in  the  drop  of  reagent,  and  on  the  edge  an 
opaque  brown  zone  of  the  sodium  double-salt,  which  permeates 
the  whole  drop  if  the  percentage  of  sodium  is  large  ;  with 
600  diameters  this  zone  is  shown  to  be  composed  of  minute 
white  transparent  granules.  If  the  mineral  contains  potassium 
also,  a  coarsely  granular  gray  zone  of  the  potassium  double-salt 
is  formed  in  the  centre  of  the  drop,  which  is  made  up  from 
granules  and  fragments  similar  to  potato-starch.  In  lower 
percentages  of  the  alkali  metals  in  the  mineral  the  phenomena 
are  more  easily  observed.  Lumps  and  short  rhombs  of  the 
potassium  double-salt,  and  acute  prisms  and  spindle-formed 
crystals  of  the  sodium  double-salt,  appear.  A  large  excess  of 
sulphuric  acid  retards  the  reaction. 

This  reaction  can  be  first  applied  for  both  alkali-metals, 
and  then  that  with  platinum  chloride  for  potassium  on  the 
same  slide,  and  finally  the  test  for  sodium  with  hydrofluosilicic 
acid  after  the  slide  has  been  prepared  with  balsam.  At  any 


62 


DETERMINATION  OF  ROCK-FORMING  MINERALS. 


rate  the  Boricky  test  for  sodium  is  to  be  preferred,  as  also  the 
test  for  potassium  with  platinum  chloride. 

Magnesium  is  shown  by  hydrogen-sodium-ammonium  phos- 
phate (microcosmic  salt).  The  drop  already  searched  for  Al 
or  K  is  saturated  with  ammonia,  a  drop  of  water  placed  at 
a  distance  of  about  i  cm.,  a  grain  of  microcosmic  salt  dissolved 
in  it,  and  the  two  drops  connected  by  a  thread  of  glass.  There 
are  immediately  formed  either  double,  forked  crystalloids 
similar  to  the  microlites  in  the  natural  glasses,  or,  if  the 
solution  is  quite  dilute,  well-defined  twins  of  hemimorphous 
crystals  of  ammonium-magnesium  phosphate  (Fig.  38).  The 
reaction  for  magnesium  often  does  not  appear  or  is  ill-defined, 


FIG.  38. — AMMONIUM-MAGNESIUM  PHOSPHATE. 
(After  Behrens.) 


FIG. 


9. — CAESIUM  ALUM. 
tfter  Behrens.) 


owing  to  an  insufficiency  of  ammonium  salts  ;  it  is  advisable, 
therefore,  to  add  a  little  hydrochloric  acid  or  ammonium 
chloride  before  saturation  with  ammonia,  o.ooi  mg.  MgO  can 
be  proved  by  this  reaction. 

Behrens  found  caesium  chloride  an  excellent  reagent  for 
Aluminium.  A  minute  portion  of  the  deliquescent  salt  is  placed 
with  the  point  of  a  platinum  wire  on  the  edge  of  the  test-drop. 
Large  transparent  octahedra(more  rarely  oo  <9oo  .  O)  of  caesium 
alum  are  immediately  formed  (Fig.  39).  If  the  solution  of  the 


METHODS  OF  INVESTIGATION.  63 

mineral  is  concentrated,  only  a  dendritic  mass  is  formed, 
and  a  small  drop  of  water  must  be  placed  beside  that  of  the 
reagent.  A  large  amount  of  sulphuric  acid  interferes  with  the 
formation  of  the  alum  crystals,  o.oi  mg.  A12O3  can  be  clearly 
proved  by  this  reaction. 

Iron  is  rarely  searched  for  with  the  microscope.  The  color 
of  the  flocculent,  fine-grained  precipitate  obtained  with  potas- 
sium ferrocyanide  from  iron  solutions  is  sufficiently  character- 
istic and  intense  when  examined  macroscopically. 

Manganese  is  proved  by  fusing  with  soda.  The  character- 
istic green  color  is  obtained  with  the  smallest  amount.  A 
microscopical  examination  is  therefore  superfluous. 

Lithium  is  precipitated  by  an  alkaline  carbonate  from  the 
solution  in  sulphuric  acid,  and  gives  well-developed  monoclinic 
crystals  of  lithium  carbonate  with  rectangular  cross-section. 
These  crystals  can  be  distinguished  from  gypsum  by  their  rect- 
angular form  and  solubility  in  dilute  sulphuric  acid  ;  from 
magnesium  double-salt  by  the  property  that  they  are  formed 
in  every  proportion  of  potassium  carbonate  and  lithium  sul- 
phate, and  remain  constant ;  while  the  crystals  of  magnesium 
double-salt  are  formed  only  in  large  excess  of  the  alkaline  car- 
bonates and  in  close  proximity,  and  soon  become  granular. 
Phosphoric  acid  seriously  retards  the  formation  of  the  lithium- 
carbonate  crystals. 

Barium  and  Strontium. — These  are  found,  together  with  cal- 
cium and  gypsum,  in  the  residue  after  lixiviation  of  the 
mass  in  the  platinum  capsule  with  water.  This  residue  is  dis- 
solved in  hot  concentrated  sulphuric  acid,  is  allowed  to  cool, 
and  is  extracted  with  water.  From  a  drop  there  is  first  a 
separation  of  barium  sulphate  in  small  lenticular,  crossed 
crystals  ;  then  of  strontium  sulphate,  at  first  in  matted  tufts 
and  fine  needles,  then  in  larger,  often  rhombic,  cruciform, 
twinned  crystals  ;  last  of  all  gypsum  separates. 

Metalloids. — Of  the  remaining  reactions  proposed  by  Behrens 


64        DETERMINATION  OF  ROCK-FORMING  MINERALS. 

for  use  with  the  rock-forming  minerals,  the  following  are  of 
importance : 

Chlorine. — The  mineral  granule  to  be  examined  for  chlorine 
is  fused  with  soda  and  decomposed  ;  a  large  quantity  of  con- 
centrated sulphuric  acid  is  added  to  the  fused  mass  in  the 
platinum  capsule,  and  the  escaping  hydrochloric-acid  gas  is 
absorbed  by  a  drop  of  water  adhering  to  the  under  side  of  a 
glass  covering  the  capsule.  This  cover  is  kept  cool  by  a  few 
drops  of  water  on  the  upper  surface.  At  the  close  of  the  pro- 
cess the  water  is  wiped  from  the  upper  surface  with  filter-paper, 
and  the  covering-glass  inverted  and  placed  on  the  stage  of  the 
microscope.  A  granule  of  thallium  sulphate  is  then  laid  in  the 
centre  of  the  drop  of  adhering  water.  Colorless  octahedra  as 
well  as  O  .  oo  O  of  thallium  chloride  are  rapidly  formed.  These 
refract  light  powerfully,  and  are  often  combined  into  clover-leaf 
trillings  and  fourlings.  0.004  mg-  NaCl  can  be  thus  proved. 

Phosphorus  and  Sulphur  can  be  proved  by  reversing  the 
reactions  already  described  for  aluminium  (for  S)  and  for  mag- 
nesium (for  P).  Insoluble  sulphates  and  phosphates  must  be 
fused  with  soda,  and  the  pulverized  fused  mass  lixiviated  with 
water.  For  proving  the  presence  of  sulphur  a  drop  of  this 
solution  is  placed  near  a  drop  of  solution  of  aluminium 
chloride  and  hydrochloric  acid  with  a  little  caesium  chloride. 
The  two  drops  are  then  united  by  a  glass  thread,  when  the 
caesium-alum  octahedra  are  developed,  as  before,  near  the  ex- 
tremity. A  concentrated  solution  of  ammonium  chloride  and 
magnesium  sulphate  is  used  as  a  reagent  for  the  detection  of 
phosphorus. 

Fluorine. — The  mineral  containing  fluorine  is  dissolved  in 
concentrated  sulphuric  acid,  and  the  escaping  gas  is  absorbed  by 
dilute  sulphuric  acid.  Such  minerals  as  topaz  or  tourmaline  must 
be  first  fused  with  twice  their  volume  of  soda  in  order  to  change 
the  fluorine  into  hydrofluosilicic  acid ;  powdered  sand  is  some- 
times added.  A  drop  of  the  sulphuric  acid  is  placed  on  the 


METHODS  OF  INVESTIGATION.  6£ 

convex  surface  of  the  platinum  cover,  and  is  then  laid  on  the 
platinum  capsule  with  the  drop  downward,  the  upper  surface 
being  cooled  as  in  the  chlorine  reaction.  The  capsule  is  gently 
warmed,  and  at  the  close  of  the  distillation  the  water  used  for 
cooling  is  removed  by  filter-paper,  and  the  drop  of  acid  con- 
taining the  fluorine  is  transferred  to  a  slide  coated  with  Canada 
balsam,  or  a  leaflet  of  barite.  (In  order  to  avoid  spurting  it  is 
advisable  to  heat  the  test,  first  fused  with  soda,  with  acetic 
acid,  and  evaporate  before  using  the  sulphuric  acid.)  A  grain 
of  sodium  chloride  is  then  added  to  the  transferred  drop.  At 
first  six-leaved  rosettes,  and  later  hexagonal  tablets,  oo  P  .  oP, 
and  short  columns,  oo  P  .  P,  of  sodium  silico-fluoride  will  appear. 
0.0036  mg.  fluorine  can  be  thus  detected. 

Silicon  and  Boron. — Their  determination  is  precisely  the 
same  as  fluorine,  except  that  hydrofluoric  acid  must  be  used 
with  the  sulphuric.  If  only  one  of  the  two  elements  is  to  be 
proved,  sodium  chloride  is  again  used  as  the  reagent ;  the  hexag- 
onal tablets  already  mentioned  are  again  formed.  If,  however, 
boron  as  well  as  silicon  is  to  be  detected,  potassium  chloride  is 
used.  Potassium  silico-fluoride  crystallizes  in  the  regular  system, 
as  O  and  O  .  oo  <9oo  ;  while  potassium  boro-fluoride  (Fig.  37,  b) 
appears  in  lance-shaped  leaves  and  in  rhombs  with  obtuse  an- 
gles, often  replaced  by  edges.  The  silico-fluorides  separate 
first.  If  the  mineral  under  examination  is  rich  in  silicon,  the 
greater  part  of  the  silicon  must  be  removed  before  the  pres- 
ence of  boron  can  be  accurately  proved.  The  mineral  powder 
mixed  with  hydrofluoric  and  sulphuric  acids  must  be  warmed 
until  the  greater  part  of  the  silico-fluoride  is  driven  out,  which 
is  absorbed  by  the  diluted  sulphuric  acid  and  tested  for  silicon 
with  sodium  chloride.  Hydrofluoric  acid  is  again  added  to  the 
mineral  test  and  again  heated  until  the  white  fumes  of  sulphu- 
ric acid  appear.  The  distillate  is  warmed  to  about  120°  C,  and  a 
drop  of  water  is  added  to  the  residue,  which  is  transferred  to  a 
slide  and  tested  for  boron  with  potassium  chloride.  The  rhom- 


66         DETERMINATION   OF  ROCK-FORMING  MINERALS. 

bic  crystals  of  potassium  boro-fluoride  are  formed  only  when 
the  drop  has  dried. 

Water. — The  water-determination  is  carried  out  with  mi- 
nute mineral  particles  in  the  same  way  as  in  blowpipe  analysis. 
Behrens  recommends  the  following  small  apparatus  for  this 
purpose  :  A  small  tube  about  10  mm.  long  and  3  mm.  in  diame- 
ter is  drawn  out  at  one  end  to  a  thread  about  2  cm.  in  length 
and  0.5  mm.  in  diameter;  after  a  gentle  heating  of  the  whole 
tube  and  drawing  through  of  air  it  is  closed.  While  the  tube 
is  yet  warm,  the  mineral  granule  is  introduced  and  the  tube 
drawn  out  to  about  half  its  length  and  melted  at  the  other  end 
also,  making  it  blunt.  The  capillary  end  is  then  cooled  by 
alcohol,  or  is  heated  to  glowing  if  no  deposition  has  taken 
place.  Such  a  deposition  of  water  then  generally  occurs,  which 
collects  in  the  capillary  portion  without  artificial  cooling. 

By  the  application  of  the  delicate  method  of  Behrens  we 
are  in  position  to  determine  immediately  with  ease  and  per- 
fect accuracy  those  most  important  elements  of  the  rock-form- 
ing minerals,  potassium,  calcium,  magnesium,  and  aluminium  ; 
the  Boricky  method  appears  to  be  more  characteristic  and  ac- 
curate for  sodium.  Rosenbusch  recommends  the  flame-reac- 
tion when  the  amount  of  sodium  is  very  small. 

C.    Mechanical  Separation  of  the  Rock-forming 
Minerals. 

THOULET.     Bull,  de  la  Soc.  mineralog.  de  France,  1879,  H.  p.  17  and  189. 
Fouqufi  ET  MICHEL  LEVY.     Mineralogie  micrographique,  p.  114. 
GOLDSCHMIDT.     N.  Jahrb.  f.   Mineralogie  und  Geologic,   1881,   i.   Beilagebd. 

p.  179- 

K.  OEBBEKE.     Ebenda,  p.  454. 
E.  COHEN  u.  L.  v.  WERVEKE.     N.  Jahrb.  f.  min.  u.  Geol.,   1883,  II.  Bd.  p. 

86-89. 
D.  KLEIN.     Bull,  de  la  Soc.  miner,  de  France,  Juin  1881,  4.  p.  149,  and  Zeitschr. 

f.   Krystallographie  und  Mineralogie  v.    Groth,    VI.   1882,  p.  306,  or  N. 

Jahrb.  f.  Min.  u.  Geol.  1882,  II.  Bd.  Ref.  p.  189. 


METHODS  OF  INVESTIGATION.  67 

P.  GISEVIUS.      Beitrage  z.  Methode  d.  Bestimmung  d.  spec.  Gew.  v.  Min.  u.  d. 

mechanischen    Trennung   von   Mineralgemengen.       Inaug.-Diss.    Univ. 

Bonn,  1883. 
C.  ROHRBACH.     N.  Jahrb.  f.   Min.  u.  Geol.    1883,   II.   Bd.   p.   186,  and  Wiede- 

mann's  Annalen  f.  Physik  u.  Chemie. 
P.  MANN.     N.  Jahrb.  f.  Min.  u.  Geol.  1884,  II.  p.  175. 

In  order  to  institute  a  quantitative  chemical  analysis  of  the 
several  rock-forming  minerals,  they  must  be  separated  as  per- 
fectly as  possible  from  each  other ;  a  partial  separation  of  the 
minerals,  as  already  stated,  is  possible  by  treatment  with  dif- 
ferent acids  and  with  the  magnet ;  but  the  separation  is  best 
effected  by  taking  advantage  of  the  relative  specific  gravities 
of  the  minerals.  Solutions  of  high  specific  gravities  are  best 
adapted  to  this  purpose,  as  by  dilution  of  the  solution  it  can 
be  lowered  easily.  This  method  of  the  mechanical  separation 
of  the  rock-constituents  has  the  additional  advantage  that  their 
specific  gravities  can  be  exactly  determined  at  the  same  time, 
and  thus  a  further  vantage-ground  for  the  determination  of  the 
mineral  be  won. 

The  solutions  at  present  known  and  universally  applied  to 
the  mechanical  separation  and  determination  of  the  specific 
gravities  are : 

I.  The  solution  of  iodides  of  potassium  and  mercury  with 
a  highest  specific  gravity  of  3.196  (Thoulet-Goldschmidt). 

II.  The  solution  of  cadmium  boro-tungstate  with  a  specific 
gravity  of  3.6  (Klein). 

III.  The  solution  of  iodides  of  barium  and  mercury  with  a 
specific  gravity  of  3.588  (Rohrbach). 

I.    SEPARATION  WITH  THE  SOLUTION  OF  THE  IODIDES  OF 
POTASSIUM  AND  MERCURY. 

Preparation  and  Properties  of  the  Solution. — Potassium  iodide 
and  mercuric  iodide  are  weighed  out  in  proportion  of  I  :  1.239 ; 
both  portions  are  thrown  into  a  large  evaporating-dish,  mixed, 


68         DETERMINATION  OF  ROCK-FORMING  MINERALS. 

and  dissolved  in  as  little  water  as  possible.  The  solution  is 
then  evaporated  on  the  water-bath  until  a  piece  of  mineral, 
tourmaline  e.g.,  sp.  gr.  3.1,  floats  upon  it ;  the  dish  is  then  re- 
moved from  the  water-bath  and  allowed  to  cool,  when  the  mass 
thickens  and  the  maximum  of  specific  gravity  is  reached.  Gen- 
erally acicular  crystals  of  a  hydrous  double  iodide  of  potassium 
and  mercury  separate  from  the  concentrated  solution  during 
the  process  of  cooling ;  this  precipitate  can  be  dissolved  in  a 
few  drops  of  water,  or  can  be  filtered  off  if  there  is  an  abun- 
dance of  the  solution.  The  salt  thus  removed  by  filtration  can 
be  redissolved  in  water  and  evaporated  to  the  required  specific 
gravity.  If  too  much  potassium  iodide  was  used,  crystals  of 
the  salt  of  the  combination  oo  O  oo  .  O  will  separate  on  the  sur- 
face of  the  liquid  ;  if,  on  the  other  hand,  there  is  an  excess  of 
mercuric  iodide,  a  thick  felt  of  yellow  needles  is  formed  which 
is  decomposed  on  dissolving  in  water,  with  the  deposition  of  a 
red  crystalline  powder  HgI2,  but  which  dissolves  in  potassium- 
iodide  solution  without  decomposition.  The  concentrated  so- 
lution is  often  decomposed  on  adding  water  with  deposition  of 
the  red  powder,  which  is,  however,  again  redissolved  on  agitat- 
ing the  solution.  The  specific  gravity  of  the  solution  changes 
on  long  standing ;  this  depends  on  the  temperature  and  moisture 
of  the  atmosphere;  the  solution  is  also  decomposed  by  organic 
substances,  as  filter-paper,  etc.  The  highest  attainable  specific 
gravity  of  the  solution  is  3.196  (Goldschmidt). 

Determination  of  the  Specific  Gravity  of  Minerals  and  Rocks 
by  the  Solution. — The  specific  gravity  of  all  those  minerals  un- 
der 3.196  can  be  determined  by  means  of  this  solution  in  the 
following  manner :  The  fragments  of  the  mineral  or  rock, 
washed  in  pure  water  and  dried,  are  thrown  into  a  tall  slim 
beaker-glass  filled  with  the  solution  at  its  maximum  density ; 
the  liquid  is  then  diluted  with  water,  or  diluted  solution,  until 
the  mineral  is  completely  suspended  in  the  solution,  i.e.  neither 
sinks  nor  rises.  The  solution  is  then  poured  into  a  25-cc.  kolben 


METHODS  OF  INVESTIGATION.  69 

accurately  calibrated,  and  filled  exactly  to  the  mark — the  mark 
had  best  be  on  the  under  side  of  the  meniscus.  The  excess  of 
liquid  is  removed  either  with  a  capillary  pipette  or  filter-paper. 
The  filled  kolben  is  weighed  and  then  emptied  back  into  the 
beaker-glass  and  the  solution  tested  with  the  fragment  of  mine- 
ral ;  the  kolben  is  refilled  to  the  mark  and  weighed,  and  the 
operation  repeated  for  a  third  time.  A  mean  is  taken  of  these 
three  weighings.  The  weighings  need  not  be  perfectly  exact 
(i.e.  to  a  few  milligrams),  varying  often  between  10  and  20  milli- 
grams, as  the  error  is  lessened  by  the  triple  weighing.  Deter- 
minations of  specific  gravity  by  this  method  are  carried  with 
accuracy  to  the  third  decimal  place.  E.g.,  quartz  and  a  25-cc. 
koiben  gave : 

First  weighing  =  77.981  grams. 
Second        "          =  77.919       " 
Third        "          =  77-973       " 


Mean         =  77.957       « 
-  Kolben      =  11.682       " 

66.275       " 
66.275  -v-  25  =  2.654  specific  gravity. 

Such  determinations  can  be  made  much  more  rapidly  and 
as  accurately,  according  to  the  principle  of  Mohr,  on  a  balance 
constructed  by  G.  Westphal  of  Celle  (price,  45  marks).  By 
this  method  the  specific  gravity  is  read  directly  on  the  balance- 
beam  after  a  single  weighing  and  with  weights  in  rider  form. 

It  must  be  noted  that  specific-gravity  determinations  of 
mineral  powder  cannot  be  made  with  the  solution,  and,  as  is 
well  known,  that  decompositions  or  inclosures  may  lower  or 
raise  the  specific  gravity  of  minerals. 

Separation  of  the  Rock-components  by  means  of  the  Solution. 
— In  order  to  separate  the  components  of  a  rock  from  each 
other,  the  rock  must  be  pulverized ;  this  should  be  preceded 


DETERMINATION  OF  ROCK-FORMING  MINERALS. 


by  an  orientation  concerning  its  probable  mineralogical  com- 
position by  an  investigation  of  a  thin  section.  The  powder  is 
then  passed  through  sieves  of  different  mesh,  and  that  part  se- 
lected for  separation  which  the  microscopical  examination  has 
demonstrated  to  be  homogeneous,  i.e.,  wherein  several  minerals 
do  not  cohere.  The  very  finest  flour-like  powder  cannot  be  used 
for  the  separation,  as  it  mixes  with  the  syrupy  solution  to  form 
a  thin  mud  ;  the  minute  mineral  particles,  grains,  and  crystals, 
therefore,  which  constitute  for  the  most  part  the  micro-crystal- 
line or  porphyritic  rocks  cannot  be  separated  by  this  method. 

If  the  rock  is  very  coarsely  granular,  it  is  often  of  great 
advantage  if  the  minerals  distinguished  by  the  pocket-lens, 
broken  away  and  dissociated,  are  separated  by 
means  of  the  solution  ;  e.g.,  the  white  feldspars 
or  the  black  bisilicates.  Mica  can  be  obtained 
pure  by  allowing  the  mineral  powder  simply  to 
slide  over  rough  paper. 

The  granular  powder  obtained  in  this  manner 
is  poured  into  an  apparatus  filled  with  the  po- 
tassium-mercury-iodide solution  at  its  highest 
specific  gravity. 

Apparatus. — As  the  very  simplest  piece  of 
apparatus  and  one  especially  adapted  to  the  pur- 
pose, an  ordinary  large  glass  separating-funnel, 
or  the  pear-shaped  vessel  described  by  T.  Harada, 
is  to  be  recommended.  This  latter  apparatus  is 
closed  above  with  a  ground-glass  stopper,  and 
terminates  in  a  narrow  tube  below,  also  provided 
with  a  ground-glass  stop-cock  (Fig.  40).  The  so- 
lution after  the  powder  is  added  to  it  is  well 
stirred  with  a  glass  rod  and  allowed  to  settle ; 


IN(CopyAfromS' 
K.  Oebbeke.) 


those  minerals  possessing  a  higher  specific  gravity  than  the  solu- 
tion sink  to  the  bottom,  and  can  be  removed  by  carefully  open- 
ing the  lower  glass  cock  after  the  solution  has  had  time  to  clear. 


METHODS  OF  INVESTIGATION,  Jl 

The  potassium-mercury  solution  is  then  diluted  by  care- 
fully dropping  distilled  water  accompanied  by  constant  agita- 
tion of  the  liquid  with  a  stirring-rod  until  another  portion  of 
the  powder  has  either  settled  to  the  bottom  or  is  suspended 
in  the  liquid.  Care  must  be  taken  that  particles  of  the  powder 
do  not  cling  to  the  rod  itself  or  the  walls  of  the  funnel. 

In  order  to  establish  the  specific  gravity  of  the  solution,  and 
consequently  of  the  precipitated  mineral  granules,  either  a 
direct  specific-gravity  determination  is  made  with  the  Mohr- 
Westphal  balance,  whereby  a  short  and  broad  glass  tube  is 
thrust  into  the  solution  and  the  plunger  of  the  balance  sunk 
inside  of  it  (a  device  preventing  a  large  loss  of  mineral  by  ad- 
hesion), or  the  so-called  indicators  are  employed.  A  series  of 
larger  mineral  fragments  of  a  known  specific  gravity  and  vary- 
ing from  i  to  3.2  is  used  for  this  purpose.  A  large  number  of 
such  minerals,  especially  of  those  with  a  specific  gravity  2-3.2, 
should  be  at  hand.  By  the  use  of  several  of  these  indicators, 
selected  according  to  the  mineral  composition  of  the  rock  as 
established  by  the  microscope,  the  specific  gravity  of  the  solu- 
tion can  be  determined  easily. 

After  removing  the  powder  which  has  fallen  to  the  bottom, 
the  solution  is  again  diluted,  and  the  operation  repeated.  The 
separated  powder  is  well  washed  with  water.  The  washings  can 
be  evaporated  with  the  diluted  solution  on  the  water-bath  until 
the  maximum  density  is  again  reached. 

The  rock-powder  can  thus  be  divided  into  portions  of 
different  known  specific  gravity  which  are  partly  pure,  i.e., 
contain  fragments  made  up  of  one  and  the  same  mineral,  or, 
if  the  rock  was  too  coarsely  pulverized,  show  the  so-called 
intermediary  products,  impurities  resulting  from  the  inter- 
penetrations  of  several  minerals.  In  the  latter  case  these  por- 
tions must  be  more  finely  pulverized  and  again  separated. 

Example. —  Tonalite. — The  microscopical  examination  of  this 
coarse-grained  rock  developed  as  components :  plagioclase 


72         DETERMINATION  OF  ROCK-FORMING  MINERALS. 

predominating,  orthoclase  subordinate,  much  quartz,  green 
hornblende,  brown  biotite,  and  magnetite,  ilmenite,  and  garnet 
as  accessory. 

The  biotite  is  first  of  all  slid  off  on  rough  paper,  and  thus 
obtained  quite  pure.  The  magnetite  can  be  withdrawn  by  a 
magnet.  The  residual  powder  is  then  thrown  into  the  solution, 
when  the  garnets  and  titanic  iron  sink  to  the  bottom.  Horn- 
blende, orthoclase,  and  quartz  fragments  are  selected  as  indi- 
cators, as  the  specific  gravities  of  the  minerals  to  be  separated 
lie  between  them.  By  slow  dilution  of  the  potassium-mercury 
solution  the  hornblende  will  be  first  precipitated,  and  only  a 
white  powder  will  remain.  The  plagioclase  will  first  precipitate 
from  this  white  powder,  then  quartz,  and  finally  the  orthoclase. 

If  the  specific  gravity  of  the  solution  was  determined  while 
the  plagioclase  was  suspended,  and  it  was  found  to  be  2.67, 
the  value  shows  that  the  plagioclase  is  an  andesine. 

Finally,  optical  investigations  can  be  instituted  on  cleavage- 
fragments  of  andesine  and  hornblende  selected  from  the 
separated  mineral  particles.  The  plagioclase  also  can  be  sub- 
jected to  a  quantitative  chemical  analysis  after  the  purity  of 
the  powder  is  established. 

Precautionary  Rules  in  Working  with  the  Potassium-Mercury 
Solution, — I.  A  large  loss  of  'the  solution  should  be  guarded 
against,  because  of  the  cost  of  preparation.  All  scattered 
drops,  residues,  and  washings  from  the  apparatus  should  there- 
fore be  gathered,  and  this  dilute  solution  again  evaporated  on 
the  water-bath.  2.  The  solution  is  very  poisonous  and  attacks 
tJie  skin. 

Regeneration  of  the  Solution. — The  solution  changes  to  a 
dark  or  reddish  brown  after  long  usage,  owing  to  the  separa- 
tion of  free  iodine.  The  iodine  is  removed,  as  L.  v.  Werveke 
has  recommended,  by  addition  of  mercury  and  agitation  of  the 
cold  solution  ;  or,  better,  by  concentrating  the  solution  on  the 
water-bath  with  constant  agitation,  and  consequent  division  of 


METHODS  OF  INVESTIGATION.  73 

the  mercury.  The  solution  again  assumes  a  honey-yellow  color, 
and  can  be  raised  to  its  highest  specific  gravity  without  injury. 
The  free  iodine  combines  with  the  mercury  to  form  the  sub- 
iodide,  which  precipitates  as  a  dirty-green  dust  on  the  mercury. 
This  is  again  changed  to  metallic  mercury  and  mercuric  iodide 
on  concentrating  the  solution,  and  is  dissolved  by  the  excess 
of  potassium  iodide  which  caused  the  separation  of  the  free 
iodine. 


II.  KLEIN'S  SOLUTION. 

D.  Klein  has  recommended  a  solution*  of  boro-tungstate  of 
cadmium  (9\VO3 .  B2O3 .  2CdO,  2HaO  +  16  aq.)  for  the  separa- 
tion of  the  rock-forming  minerals.  Although  the  preparation 
of  this  solution  is  far  more  complicated  than  that  of  the 
potassium-mercury  iodides,  yet  it  is  to  be  preferred,  as  nearly 
all  of  the  rock-forming  minerals  can  be  separated  by  it,  owing 
to  its  high  specific  gravity — 3.6 ;  while  many  minerals,  and  the 
most  important,  as  augite,  hornblende,  olivine,  etc.,  whose 
specific  gravity  lies  above  3.19,  cannot  be  separated  by  the 
solution  of  the  iodides. 

The  process  of  separation  with  Klein's  solution  is  exactly 
analogous  to  that  with  the  iodide  solution.  It  must  be  re- 
membered, however,  to  dissolve  out  with  acids  all  carbonates, 
such  as  calcite,  etc.,  from  the  rock-powder,  as  they  decompose 
the  solution.  The  apparatus,  either  separating-funnel  or 
Harada's  vessel,  must  be  surrounded  by  hot  water  or  otherwise 
warmed,,  as  the  salt  must  be  melted  at  75°  if  a  solution  with  a 
specific  gravity  of  3.5-3.6  is  desired. 

The  Preparation  of  the  solution  is  as  follows :  A  solution  of 
Na2WO4  in  five  parts  water  is  first  prepared  and  then  boiled 

*  According  to  the  author's  experience,  Klein's  solution  is  the  best  and  the  most  durable  of 
all  the  solutions  of  high  specific  gravity  used  for  the  mechanical  separation  of  the  rock-com- 
ponents. 


74         DETERMINATION  OF  ROCK-FORMING  MINERALS. 

with  1.5  parts  B(OH)3  until  the  whole  is  dissolved.  On  cooling 
and  agitating  the  solution,  crystals  of  borax  and  sodium  poly- 
borates  separate,  which  must  be  removed.  The  decanted  liquid 
is  again  evaporated,  and  the  newly-formed  crystals  removed, 
and  this  process  is  repeated  until  glass  floats  on  the  surface.  A 
boiling  solution  of  BaCl2  is  then  added  (iBaCla  :  3Na2WO4). 
A  thick  white  precipitate  is  formed,  which  is  filtered  off,  well 
washed  with  water,  and  finally  dissolved  in  dilute  HC1 
(iHCl  sp.  gr.  1. 1 8  :  ioH2O).  Hydrochloric  acid  is  added  in 
excess  to  the  solution,  and  the  whole  evaporated  to  dryness, 
when  H2WO4  separates.  The  dried  mass  is  again  dissolved  in 
hot  water,  boiled  for  about  two  hours,  water  being  added  from 
time  to  time,  and  the  H2WO4  filtered  off.  Tetragonal  crystals 
of  th^  compound  9WO3  .  B2O3 .  2BaO,  2H2O  +  19  aq.  separate 
from  the  solution,  and  these  are  purified  by  recrystallization. 
Finally,  CdSO4  is  added  to  a  boiling  solution  of  these  crystals, 
when  the  soluble  cadmium  boro-tungstate  9WO3 .  B2O3 .  2CdO, 
2H2O  +  16  aq.  is  formed  and  filtered  from  the  insoluble  BaSO4. 

Cadmium  boro-tungstate  dissolves  in  less  than  T^  of  its 
weight  of  water ;  it  crystallizes  on  evaporation  on  the  water- 
bath  and  cooling.  The  solution  of  these  crystals  has  a  specific 
gravity  of  3.28  at  15°  C. 

Evaporation  of  the  solution  must  be  done  always  on  the 
water-bath ;  if  a  specific  gravity  of  3.6  is  desired,  the  solution 
is  evaporated  until  olivine  floats,  and  is  then  allowed  to  stand 
24  hours.  Crystalline  masses  are  deposited  which  are  removed 
from  the  solution,  purified  and  melted  at  75°  in  the  separat- 
ing-apparatus,  placed  either  over  the  water-bath  or  in  a  jacket 
filled  with  hot  water.  Spinel  floats  on  this  fused  mass. 

Cadmium  boro-tungstate  solution  can  be  obtained  from 
chemical  depots  ready  for  use. 

In  addition  to  its  higher  specific  gravity  this  solution  has 
these  further  advantages  over  the  potassium-mercury  solution : 
it  is  non-poisonous,  does  not  attack  the  skin,  and  remains  at  a 


METHODS   OF  INVESTIGATION'.  f$ 

constant  specific  gravity;   carbonates  and  metallic  iron,  how- 
ever, decompose  it. 


III.    ROHRBACH'S  SOLUTION  OF  THE  IODIDES  OF  BARIUM 

AND  MERCURY. 

This  is  even  more  valuable  than  Klein's  solution  for  the 
separations.  The  specific  gravity  of  the  concentrated  barium- 
mercury  solution  is  nearly  the  same  as  Klein's,  but  the  prep- 
aration is  not  so  complicated ;  moreover  no  decomposition  is 
effected  by  the  carbonates. 

The  solution  of  the  Iodides  of  Mercury  and  Barium  is  pre- 
pared in  the  following  manner :  100  parts  of  barium  iodide  and 
130  parts  of  mercuric  iodide  are  weighed  as  rapidly  as  possible  ; 
both  in  powdered  form  are  transferred  to  a  dry  kolben  over 
an  oil-bath  heated  to  about  200°  C,  are  well  shaken  together, 
and  dissolved  in  about  20  kcm.  of  water.  The  solution  is 
hastened  by  whirling  the  contents  with  a  glass  rod  bent  at  the 
lower  extremity.  If  all  is  dissolved,  the  solution  is  allowed  to 
boil  a  little,  and  is  then  transferred  to  a  water-bath,  where  it  is 
evaporated  until  a  fragment  of  epidote  floats.  On  allowing  the 
solution  to  cool,  the  specific  gravity  increases  until  olivine 
floats  ;  a  double-salt,  however,  is  deposited,  which  is  allowed  to 
settle  at  the  bottom  of  a  tall  beaker-glass,  and  is  removed  by 
careful  decantation  of  the  clear  solution.  Filtration  is  not  ad- 
visable, as  filter-paper  cannot  be  used.  The  solution  thus  pre- 
pared attains  a  specific  gravity  of  3.5/5-3.588. 

The  method  of  operation  with  this  solution  is  exactly  the 
same  as  with  the  potassium-mercury  solution,  except  that 
the  barium-mercury  solution  must  not  be  diluted  with  water, 
but  always  with  dilute  solution.  This  latter  solution  can  be 
obtained  easily  by  allowing  a  layer  of  water  to  stand  for  about 
24  hours  on  the  concentrated  solution  in  a  beaker-glass,  when 


76         DETERMINA  TION  OF  ROCK-FORMING  MINERALS. 

the  mixing  will  follow  by  diffusion.  Red  mercuric  iodide  gen- 
erally deposits  on  diluting  with  water.  The  powder  to  be 
separated  must  be  perfectly  dry ;  iodide  of  potassium  must  be 
used  at  first  in  washing,  which  redissolves  any  precipitated 
mercuric  iodide. 

Rohrbach  recommended  also  that  the  separation  of  all 
minerals  below  3.1  should  be  carried  out  with  the  potassium- 
mercury  solution,  and  that  the  further  separation  of  the 
heavier  minerals  of  sp.  gr.  3.1-3.58  should  be  prosecuted  with 
the  barium-mercury  solution ;  closed  apparatus  for  separation, 
as  Harada's,  is  also  advisable.  On  continued  standing  (i.e.,  for 
several  months)  the  solution  becomes  specifically  lighter,  ow- 
ing to  the  deposition  of  mercuric  iodide ;  it  cannot,  therefore, 
be  employed  in  separating  minerals  of  sp.  gr.  3.2-3.6. 


IV.  METHODS  OF  SEPARATION  BASED  ON  THE  DIFFERENT 
ACTION  OF  ACIDS  ON  MINERALS. 

ZlRKEL  Und  ROSENBUSCH,  1.   C. 

F.  FOUQUE  et  MICHEL  LEVY.    Mineralogie  micrographique.    Paris,  1879,  P- II6- 

F.  FOUQUE.  Nouveaux  precedes  d'analyse  mediate  des  roches  et  leur  applica- 
tion aux  laves  de  la  derniere  eruption  de  Santorin.  Mem.  savants 
etrangeres  de  1' Academic  des  sciences.  Paris,  XXII.  p.  n,  and  Compt. 
rend^i874,  p.  869., 

K.  OEBBEKE.     N.  Jahrb.  f.  Min.  u.  Geol.  1881,  I.  Beilagebd.  p.  455. 

A.  CATHREIN.     Ebenda,  1881.  I.  Bd.  p.  172. 

It  has  been  hinted  already  that  a  basis  for  the  more  exact 
determination  of  many  minerals  can  be  obtained  in  many  cases 
by  simple  treatment  of  the  powdered  rock  with  various  acids. 
With  this  in  view,  a  thin  section  of  the  rock  is  first  examined 
in  order  to  gain  some  idea  of  its  mineralogical  composition. 
Small  fragments  of  the  rock  are  then  finely  powdered  and 
treated  with  concentrated  hot  hydrochloric  acid  in  a  beaker- 
glass.  Any  evolution  of  gas  must  be  carefully  noted,  or  forma- 


METHODS  OF  INVESTIGATION.  77 

tion  of  any  precipitate,  especially  separation  of  sulphur  or 
silicic  acid.  The  acid  is  generally  allowed  to  act  on  the  powder 
for  some  hours,  and  is  then  filtered.  The  sulphur  is  then  dis- 
solved from  the  dried  powder  on  the  paper  with  carbon  disul- 
phide  or  ether,  and  the  silicic  acid  by  boiling  in  sodium  car- 
bonate. The  powder  is  then  thoroughly  washed,  dried,  mixed 
with  Canada  balsam,  and  suitably  prepared  on  a  slide  for  a 
microscopical  examination.  If  it  is  evident  that  one  or  more 
of  the  rock-forming  minerals  has  dissolved,  the  ordinary  quali- 
tative chemical  analysis  of  the  filtrate  is  set  in  course. 

The  following  rock-forming  minerals  are  soluble  in  hydro- 
chloric acid : 

I.  Soluble  without  evolution  of  gas   or  separation  of  a 
precipitate : 

Magnetite,   Hematite,   Apatite   (P2OB),    Titaniferous 

Magnetite  (difficultly  soluble). 
II.  Soluble  with  evolution  of  CO2 : 

Calcite,  Aragonite  (Ca),  Dolomite  (CaMg),  Magnesite 
(difficultly  soluble),  Siderite  (Fe). 

III.  Soluble  with  separation  of  S  : 

Pyrrhotite,  Pyrite  (difficultly  soluble). 

IV.  Soluble  with  separation  of  pulverulent  SiO2 : 

Leucite  (K),  Meionite  (Ca),  Scapolite  (Ca,  Na),  Labra- 
dorite    and   Bytowniie  (more   difficultly  soluble, 
Ca,  Na),  Anorthite  (Ca). 
V.  Soluble  with  separation  of  gelatinous  SiO2 : 

Sodalite  (Cl),  Hauyn  and  Nosean  (SO,),  Ncpheline 
(Na),  Wollastonite  (Ca),  Olivine  (Mg),  Melilite  (Ca), 
nearly  all  Zeolites,  Serpentine,  then  Chlorite  and 
Epidote  (difficultly  soluble). 

Exact  determinations  cannot  be  carried  out  by  this  method, 
and  all  the  less  because  many  minerals,  and  those  too  the 


78         DETERMINATION  OF  ROCK-FORMING  MINERALS. 

•  most  commonly-occurring  silicates,  possess  a  similar  chemical 
composition  ;  e.g.,  scapolite  or  meionite,  with  the  species  of 
plagioclase  closely  related  to  anorthite.  Such  minerals  as  the 
carbonates,  apatite  or  sodalite,  can  be  more  easily  demonstrated, 
as  they  give  characteristic  reactions.  If  hydrochloric  acid  of 
different  degrees  of  concentration  be  used,  more  exact  results 
are  obtained,  as  the  solubility  of  the  minerals  depends  upon 
the  size  of  the  granule,  temperature,  duration  of  action,  and 
degree  of  concentration  of  the  acid.  Unfortunately  no  careful, 
systematic  investigations  have  been  made  in  this  direction ; 
e.g.,  nepheline  and  olivine  occurring  together  in  a  nepheline 
basalt  can  be  separated  by  treatment  with  hydrochloric  acid. 

Fouque"  has  proposed  another  method  of  separation  which 
depends  upon  the  application  of  hydrofluoric  acid  of  different 
degrees  of  strength. 

Pure  concentrated  hydrofluoric  acid  is  poured  into  a  plati- 
num dish,  and  about  30  grams  of  the  powdered  rock  is  slowly 
added  and  stirred  with  a  platinum  spatula.  Nearly  all  min- 
erals except  those  containing  Fe  and  Mg  are  dissolved,  form- 
ing fluorides  and  silico-fluorides  and  a  thick  jelly  of  silicic  acid 
and  alumina.  The  different  minerals  can  be  separated  accord- 
ing to  the  duration  of  the  reaction  ;  the  amorphous  minerals 
being  decomposed  first,  then  the  feldspars,  then  quartz,  and 
finally  the  iron  silicates  and  magnetite.  If  the  action  of  the 
acid  on  a  mineral  has  been  studied  sufficiently  and  its  arrest  is 
desired,  a  strong  fine  stream  of  water  may  be  directed  into  the 
dish,  and  the  acid  thus  be  diluted  until  it  ceases  to  act  on  the 
powder.  The  gelatinous  mass  is  pressed  together,  and  washed 
with  water ;  the  unattacked  mineral  remaining  at  the  bottom 
of  the  dish. 

In  this  manner  feldspar,  e.g.,  can  be  separated  from  a  vit- 
reous mass,  or  augite  and  hornblende  from  other  components. 


METHODS  OF  INVESTIGATION*.    t  7Q 

^^/r 

^k-*-— — * 

V.  SEPARATION  OF  THE  ROCK-CONSTITUENTS  BY  MEANS  OF 
THE  ELECTRO-MAGNET. 

F.  FOUQUE.     Santorin.     Paris,  1879. 

F.  FOUQUE.     Mem.  Acad.  des  sciences,  1874,  XXII,  No.  n. 

C.  DOELTER.     Sitzungsb.  d.  k.  Akad.  d.  Wiss.  in  Wien.  LXXXV.  Bd.  I.  Abth. 

1882.  p.  47  and  442. 

C.  DOELTER.     Die  Vulcane  der  Capverden.  Graz,  1882. 
P.  MANN.     N.  Jahrb.  f.  Min.  u.  Geol.  1884.  II.  p.  181. 

It  has  been  noted  already  that  for  a  long  period  the  ex- 
traction of  magnetite  from  the  rock-powder  has  been  effected 
by  means  of  an  ordinary  powerful  magnet ;  more  recently  the 
electro-magnet  has  been  applied  to  the  separation  of  the  fer- 
riferous minerals  from  those  containing  no  iron. 

The  credit  for  its  application  to  petrographical  investiga- 
tions is  due  to  Fouque,  and  especially  that  he  called  attention 
to  its  value  in  the  mechanical  analysis  of  rocks. 

It  is  impossible  to  separate  the  components  of  a  rock  by 
use  of  the  electro-magnet  alone  ;  several  methods  must  always 
be  combined  in  order  that  the  minerals  may  be  separated  as 
pure  as  possible.  Therefore  the  solution  of  the  iodides  of 
potassium  and  mercury  is  first  advantageously  employed, 
then  Klein's  or  Rohrbach's  solution,  and  finally  the  mineral 
portions  separated  by  means  of  these  solutions  are  completely 
purified  with  the  electro-magnet.  E.g.,  it  is  required  to  separate 
the  components  of  a  phonolite — magnetite,  sanidine,  nepheline, 
and  augite.  The  magnetite  is  removed  with  the  magnetic 
needle.  In  the  residue,  sanidine  and  nepheline  are  separated 
from  the  augite  by  means  of  the  potassium-mercury  solution 
of  specific  gravity  about  3,  when  the  augite  is  obtained  very 
pure.  The.  sanidine  and  nepheline  can  be  purified  by  means 
of  the  electro-magnet,  and  the  nepheline  separated  from  the 
sanidine  (and  augite  accidentally  present)  again  by  means  of 


8O         DETERMINATION  OF  ROCK-FORMING  MINERALS. 

the  potassium-mercury  solution  ;  or  the  nepheline  can  be  dis- 
solved in  hydrochloric  acid. 

If,  on  the  other  hand,  the  components  of  a  vitreous  augite- 
andesite  are  to  be  separated,  the  vitreous  base  may  be  removed 
by  means  of  hydrofluoric  acid,  the  augite  separated  from  the 
plagioclase  by  the  electro-magnet,  and  the  varieties  of  plagio- 
clase,  in  case  several  species  are  present,  isolated  by  the  potas- 
sium-mercury solution. 

The  powder  must  be  dry  and  free  from  the  very  finest  dust 
when  the  electro-magnet  is  used.  The  size  of  the  grains 
depends  upon  the  density  of  the  rock. 

If  several  ferriferous  mineral  species  occur  in  the  rock  to  be 
examined,  e.g.,  magnetite,  ilmenite,  augite,  biotite,  olivine,  etc., 
they  can  be  separated  from  each  other  by  varying  the  strength 
of  current  passing  through  the  electro-magnet.  At  first  two 
elements  are  used,  then  four,  six,  eight,  and  finally  ten. 
Doelter  has  shown  that  the  minerals  can  be  arranged  in  the 
following  series  according  to  their  different  powers  of  being 
attracted : 

Magnetite, 

Hematite,  Ilmenite, 

Chromite,  Siderite,  Almandine, 

Hedenbergite,  Ankerite,  Limonite, 

Augite  (rich  in  iron),  Pleonaste,  Arfvedsonite, 

Hornblende,  Augite  (light-colored),  Epidote,  Pyrope, 

Tourmaline,  Bronzite,  Idocrase, 

Staurolite,  Actinolite, 

Olivine,  Pyrite,  Chalcopyrite, 

Biotite,  Chlorite,  Rutile, 

Hatty  n,  Diopside,  Muscovite, 

Nepheline,  Leucite,  Dolomite. 

Doelter  has  also  described  a  piece  of  apparatus  suitable  for 
such  separations.  In  this  the  distance  between  the  powder 


METHODS  OF  INVESTIGATION,  8 1 

lying  on  a  glass  plate  and  the  hook-shaped  poles  of  the  horse- 
shoe magnet  can  be  measured.  He  also  advised  the  prepara- 
tion of  a  scale  of  minerals  for  each  apparatus  with  its  varying 
power  of  the  current,  analogous  to  the  indicators  used  in  the 
separation  by  solutions  of  high  specific  gravity,  in  order  to 
determine  the  individual  power  of  attraction  with  the  different 
strength  of  current.  The  mineral  granules  to  be  separated 
should  be  from  0.14  to  0.18  mm.  in  diameter,  v.  Pebal  states 
that  powder  suspended  in  water  is  preferable  to  the  dry. 


D.  Explanations  of  the   Tables   relating    to  the    Mor- 
phological Properties  of  the  Rock-forming  Minerals. 

ZIRKEL.     Mikr.  Beschaff.  d.  Min.  u.  Gesteine.     Leipzig,  1873. 
ROSENBUSCH.     Mikr.  Physiogr.  d.  petrogr.  wicht.  Miner.     Stuttgart,  1873. 

E.  COHEN.     Sammlung  von  Mikrophotographien    zur  Veranschaulichung  der 

mikroskopischen  Structur  von    Mineralien    und   Gesteinen.      Stuttgart, 

1883. 

FOUQUE  ET  MICHEL  LEVY.     Mineralogie  micrographique.     Paris,  1879. 
THOULET.     Contributions  a  1'etude  des  proprietes  physiques  et  chimiques  des 

mineraux  microscopiques.     Paris,  1880. 
v.  PEBAL.     Sitzungsber.  d.  k.  k.  Akad.  der  Wiss.  math.  nat.  Cl.  1882.  p.  193. 

I.  MODE  OF  OCCURRENCE  OF  THE  ROCK-CONSTITUENTS. 

The  mineral  constituents  of  a  rock  occur  either  in  perfectly- 
developed  crystals,  often  sharply  defined,  in  crystalline  grains, 
or  as  microlites  or  crystallites. 

It  is  seldom,  however,  that  the  crystals  appearing  in  the 
rocks  are  so  large  that  the  system  of  crystallization  can  be  de- 
termined by  the  macroscopical  examination  or  measurement 
of  the  angles  alone.  In  order,  therefore,  to  determine  the 
mineralogical  composition  of  a  rock,  a  thin  section  must  be 
prepared  wherein  the  constituents,  appearing  in  the  forms  just 
mentioned,  are  in  sections  in  every  possible  direction.  In  this 


82         DETERMINATION  OF  ROCK-FORMING  MINERALS. 

case  the  determination  of  the  crystalline  form  is  rendered 
much  more  difficult,  and  is  impossible  simply  from  the  form  of 
the  cross-section.  By  suitable  combination  of  the  form  of 
cross-section,  optical  properties,  cleavage,  and  finally  by  meas- 
urement of  the  angles,  it  can  be  determined  in  most  cases  to 
which  system  of  crystallization  the  mineral  belongs.  E.g.,  a 
mineral  appears  whose  cross-sections  are  octagonal,  with  cleav- 
age at  nearly  right  angles  ;  or  are  elongated,  rectangular,  or 
hexagonal,  with  cleavage-fissures  parallel  to  the  longest  axis. 
The  mineral  could  belong  to  the  tetragonal  as  well  as  the 
rhombic  or  monoclinic  system.  The  section  must  be  examined, 
therefore,  in  parallel  and  convergent  polarized  light.  The 
form  of  the  cross-sections  shows  that  the  mineral  is  developed 
in  long  eight-sided  prisms  with  prismatic  cleavage  ;  the  octago- 
nal sections  are  the  transverse  sections  at  right  angles  to  the 
£-axis.  If  they  appear  as  isotrope  in  parallel  polarized  light 
and  show  in  convergent  polarized  light  a  fixed  axial  cross,  the 
mineral  is  tetragonal,  possibly  belonging  to  the  meionite  group. 
If,  on  the  contrary,  the  transverse  sections  as  well  as  the  lon- 
gitudinal are  anisotrope  and  develop  a  middle  line  in  con- 
verging polarized  light,  it  is  rhombic ;  and  if,  finally,  one  optic 
axis  is  visible,  it  is  monoclinic  and  the  mineral  may  be,  e.g., 
from  the  augite  group. 

By  measurement  of  the  angles  it  can,  in  the  latter  case,  still 
be  determined  which  faces  belong  to  the  prism  co  P  and  the 
pinacoids,  and  to  which  faces  the  cleavage-fissures  are  parallel. 

In  measuring  the  angle  of  cleavage  the  direction  of  the  sec- 
tion must  always  be  carefully  noted,  as  the  value  of  the  angle 
of  cleavage  varies  within  wide  limits,  according  to  the  inclina- 
tion of  the  section  to  the  chief  or  vertical  axis.  E.g.,  augite 
cannot  be  distinguished  from  hornblende  by  the  angle  of  cleav- 
age alone,  as  augite  prisms  cut  at  an  angle  of  40°  to  the  verti- 
cal axis,  following  —  2^00  in  the  zone  oP :  oo  Pco,  will  show 
a  cleavage-angle  of  124°  2',  which  lies  very  near  the  angle  of 


METHODS   OF  INVESTIGATION.  83 

a  section  of  hornblende  cut  perpendicularly  to  the  vertical 
axis. 

Thoulet  (1.  c.,  p.  28)  has  determined  the  value  of  the  cleav- 
age-angle of  augite,  hornblende,  orthoclase,  and  labradorite  for 
the  different  directions  of  the  sections  and  according  to  the  am- 
plitude of  its  inclination  to  the  vertical  axis.  The  determina- 
tion for  the  first  two  of  these  minerals  is  given  in  the  table  on 
the  following  page. 

It  is  therefore  impossible  by  observation  of  a  single  cross- 
section  with  nearly  rectangular  cleavage  to  determine  with  ac- 
curacy, for  example,  whether  the  observed  monoclinic  green 
or  brown  mineral  is  augite  or  hornblende.  Nor  less  by  simply 
proving  the  presence  or  absence  of  pleochroism.  It  is  therefore 
necessary  to  examine  a  series  of  cross-sections  of  the  particular 
mineral,  and  it  can  only  be  settled  with  any  great  accuracy 
whether  a  mineral  is  augite  or  hornblende  when  several  trans- 
verse sections  show  a  cleavage-angle  approaching  87°  or  124°. 

Often  the  shape  of  the  crystal  outline  shows  that  the  plane 
of  the  section  is  inclined  to  the  vertical  axis,  and  gives  ap- 
proximately its  angle  of  inclination  ;  if  the  constituents  are  in 
a  granular  condition,  this  mark  of  recognition  is  wanting,  and 
hence  complicates  the  determination.  The  direction  of  the 
section  can  also  be  approximately  determined  by  comparison 
of  the  optical  relations  (according  to  examinations  in  converg- 
ent polarized  light). 

The  simple  proof  of  parallel  extinction  on  one  or  a  few  sec- 
tions can  give  no  safe  conclusions  as  to  whether  the  mineral  is 
rhombic  or  monoclinic  ;  e.g.,  the  determination  of  c :  c  to  about 
20°  in  augite  and  hornblende.  As  many  observations  as  pos- 
sible, therefore,  must  be  made  on  sections  optically  oriented. 
In  the  cases  mentioned  this  is  done  most  easily  on  prismatic 
cleavage-leaflets. 

Microscopical  Measurements  of  Angles  are  made  with  the 
polarization-microscope  in  the  same  manner  as  the  determina- 


84 


DETERMINATION  OF  ROCK-FORMING  MINERALS. 


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METHODS   OF  INVESTIGATION.  85 

tion  of  the  direction  of  extinction.  The  instrument  is  ac- 
curately centred,  one  leg  of  the  angle  to  be  measured  so  dis- 
posed that  it  coincides  exactly  with  one  arm  of  the  cross- 
threads,  the  apex  of  the  angle  reaching  exactly  to  the  junction 
of  the  cross-threads  of  the  ocular.  The  position  of  the  stage  is 
read  and  the  stage  revolved  until  the  other  leg  of  the  angle 
coincides  with  the  same  arm  of  the  cross-threads,  and  its  posi- 
tion again  read  :  the  difference  of  the  two  readings  gives  the 
magnitude  of  the  angle  measured. 

If  the  rock-components  are  granular,  their  determinations 
are  greatly  complicated,  as  one  can  neither  draw  any  satisfac- 
tory conclusion  as  to  the  crystalline  form  from  the  character  of 
the  outline  merely,  nor  can  it  be  determined  to  which  faces  the 
cleavage-fissures  are  parallel ;  one  is  therefore  restricted  to 
the  determination  of  the  color,  direction  of  cleavage,  magni- 
tude of  the  cleavage-angle,  and  especially  to  the  optical  prop- 
erties of  the  mineral  granules. 

The  Microlite  is  another  form  of  development  of  the  rock- 
forming  minerals.  E.  Cohen  has  designated  as  "  microlites  " 
all  those  crystals  which  cannot  be  prepared  in  sections  in  suit- 
able positions,  generally  horizontal,  the  micas,  however,  vertical, 
appearing  in  the  thin  section  as  perfectly  developed  individu- 
als ;  it  makes  no  difference  whether  the  mineral  species  can 
be  determined  or  not.  Vogelsang  (Phil.  d.  Geol.  1867,  p.  139) 
has  recommended  that  the  term  "microlite  "  be  used  only  with 
the  acicular  microscopical  mineral  forms  without  any  regard  as 
to  whether  it  can  or  cannot  be  determined  to  which  mineral 
the  microlite  belongs.  Many  rock  forming  minerals,  as  augite, 
hornblende,  and  the  feldspars,  appear  as  microlites ;  in  the 
porphyritic  rocks  these  occur  with  larger  crystals  or  grains,  and 
thus  chronicle  their  different  stages  of  formation  or  separation. 
The  large  crystals  and  grains — the  so-called  "  springlings" 
(Einsprenglinge)  (components  of  the  first  class) — were  formed 
sooner  than  the  microlites  (components  of  the  second  class) 


86 


DETERMINATION  OF  ROCK-FORMING  MINERALS. 


of  the  same  mineral  species  forming  the  principal  ground-mass 

of  the  porphyritic  rocks. 

As  microlites,  and  nearly  always  as  such,  appear  sillimanite 

(comp.  Fig.  71),  rutile,  zircon,  commonly  tourmaline,  etc.;  while 

other  minerals,  as   olivine,  titanite,  etc.,  never  or  rarely  thus 

appear. 

The  Crystallites  (see  Fig.  41)  form  a  transition-stage  to  the 
microlites,  i.e.,  lie  between  the  amor- 
phous and  crystalline  condition. 
Vogelsang  designates  by  this  term 
"all  inorganic  products  which  show 
some  systematic  arrangement,  but 
not  the  general  character  of  crystal- 
lized bodies,  i.e.,  no  polyhedral  out- 
line." The  crystallites  exert  no  in- 
fluence on  polarized  light. 

Crystallites  occur    frequently   in 
vitreous  or  semi-vitreous  rocks.     The 

simplest  forms  are  the  Globulites,  as  those  exceedingly  minute, 

isotrope,  for  the  most  part  globular,  forms  which  have  separated 

in  the  vitreous  ground-mass  of  such 

rocks     are     designated.      If   several 

such  globulites  are  chained  together, 

the  Margarites   are    formed.     If   the 

members  of  this  chain-like  aggregate 

of  globulites  are  fused  together  into 

a    long    needle,    the    Longulites    are 

formed. 

The  Crystalloids  form  yet  another 

stage  of  transition  to  the  microlites; 

"  these  are  more  of  a  unit,  act  also 

on  polarized  light,    but  do   not   yet 

show  the  polyhedral  outline  of  the  microlite." 

The   genesis    of    the    rock-forming   minerals   is   therefore 


FIG.  41. 
CRYSTALLITES  AND  MICROLITES. 


FIG.  42. 

MICRO-FLUCTUATION  STRUCTURE. 
BELONITES  AND  TRICHITES. 


METHODS   OF  INVESTIGATION.  87 

briefly  as  follows :  The  crystallites  are  the  primitive  form, 
— the  globulitcs  being  first  in  order ;  the  crystalloids  mark  a 
further  progress  in  development;  these  form  a  transition  to 
the  microliteSj  which  in  turn  only  differ  in  size  from  the  crystals. 
Vogelsang  has  proposed  a  further  subdivision  of  the 
crystallites  and  crystalloids,  resting  upon  their  pellucidity. 
A  pellucid  species  may  be  called  a  Belonite ;  a  non-pellucid,  a 
Trichite.  (Fig.  42.) 

II.  STRUCTURE  OF  THE  ROCK-FORMING  MINERALS. 
The  following  should  be   especially  noted  concerning  the 
microscopical  relations  of  the  rock-components : 

1.  The  disturbances  in  crystallization. 

2.  The  destruction  of  crystals  already  formed. 

3.  The  concentric  structure  of  crystals. 
Disturbances  in  Crystalization  are    not    common,  and    are 

manifested  in  the  imperfect  development  of  the  crystal  at  one 
end  or  in  the  sunken  faces  whereby  the  crystals  take  on  an 
"  etched  appearance ;"  the  phenomenon  so  often  noticed  in 
magnetite  is  also  to  be  mentioned — the  regular  grouping  of 
several  small  crystals  in  three  directions  at  right  angles  to  each 
other  corresponding  to  the  axes,  thus  forming  the  outline  to 
a  larger  crystal. 

Imperfectly-developed  crystals  occur  on  one  termination  ; 
e.g.,  on  hematite,  where  hexagonal  tablets  are  notched  and 
lapped  on  one  or  two  sides,  or  on  the  crystals  of  hornblende, 
augite,  etc.,  which  are  often  covered  at  one  end  with  several  sub- 
individuals  and  thus  acquire  an  appearance  resembling  a  ruin. 

On  olivine,  leucite,  etc.,  often  occur  crystals  with  faces  de- 
pressed in  consequence  of  the  interrupted  development.  In  a 
word,  exactly  the  same  phenomena  of  growth  and  disturbance 
are  noticed  in  the  crystals  separated  from  the  molten  rock- 
magma  as  can  be  perceived  on  crystals  formed  from  a  solution. 
The  destruction,  fracture,  and  bruising  of  crystals  already  fully 


88         DETERMINATION  OF  ROCK-FORMING  MINERALS. 


FIG.  43. 

CORRODED  QUARTZ  CRYSTAL. 
(After  Fouqu^.) 


formed  can  be  commonly  observed  on  the  microscopic  con- 
stituents of  the  more  recent  and  vit- 
reous rocks,  just  as  the  same  phe- 
nomena are  observed  on  the  macro- 
scopic individuals ;  e.g.,  of  tourma- 
line, epidote,  etc.  The  larger  mineral 
components  which  first  separated 
show  such  fractures  especially.  These 
are  a  direct  consequence  of  the  pres- 
sure which  the  molten,  fluctuating 
rock-magma  exerted  on  the  crystals 
already  formed,  if  any  change  in  the 
rapidity  of  fluctuation  was  induced 
by  any  obstruction ;  e.g.,  another  opposing  large  crystal  lying 
in  the  immediate  vicinity.  The  corresponding  fragments  of 
the  crystal,  as  well  as  the  crystal  or  other  matter  causing  the 
fracture,  can  be  observed  very  often  lying  close  together.  Such 
fractures  are  for  the  most  part  restricted  to  the  thin  tabular  or 
long  acicular  crystal  individuals  ;  they  are  observed,  therefore, 
most  commonly  on  the  feldspars,  augite  or  hornblende  crystals, 
while  the  micas  because  of  their  elasticity  show  only  a  bending 
or  exfoliation.  However,  quartz  grains  and  crystals  often  ap- 
pear shattered  into  small  splinters  and  plates. 

The  Destruction  of  Crystals  already  Formed. 

The  larger  crystalline  components  undergo  further  changes' 
through  the  caustic  action  of  the  liquid  magma,  as  manifested 
in  the  corrosion,  partial  fusion,  and  even  total  destruction  of 
the  crystal.  Thus  quartz  occurring  in  the  porphyritic  eruptive 
rocks  often  shows  a  sinus-like  penetration  of  the  ground-mass. 
(Fig.  43.)  Leucite  and  olivine  as  well  as  augite  crystals  or  grains 
often  show  an  etched  surface,  sometimes  covered  with  regular 
depressions,  probably  caused  by  the  caustic  action  of  the  mag- 
ma on  the  crystals  for  a  long  period,  similar  to  the  figures  and 


METHODS   OF  INVESTIGATION.  89 

depressions  often  formed  on  artificial  crystals  by  action  of  the 
mother-liquor. 

If  action  of  magma  on  the  crystals  already  formed  was 
more  powerful,  a  partial  fusion  ensued,  as  may  be  observed 
very  often  on  crystals  of  feldspar  or  augite  of  the  eruptive 
rocks,  where  some  faces  are  yet  more  or  less  evident. 

The  resolution  of  the  edges  into  minute  crystals  and  grains 
as  is  often  observed  on  the  larger  olivine,  augite,  and  feldspar 
crystals  is  another  remarkable  corrosion-phenomenon,  depend- 
ing upon  this  action  of  the  magma.  The  minute  crystals  are 
to  be  regarded  as  newly-deposited  crystals  of  the  same  mineral, 
and  the  grains  as  separated  particles.  The  diopside,  bronzite, 
and  olivine  grains  of  the  so-called  "  olivine  lumps'*  in  the 
basalts  often  show  such  changes.  More  remarkable  yet  is  that 
on  the  omphacite  of  eclogite,  a  rock  classed,  however,  accord- 
ing to  its  formation  with  the  crystalline  schists. 

Another  change  also  ascribed  to  the  action  of  the  molten 
magma,  and  commonly  observed  on  hornblende  and  biotite 
crystals  of  the  more  recent  eruptive  rocks  richer  in 
iron,  consists  of  the  appearance  of  an  opaque  margin 
(Fig.  44).  The  crystals  are  surrounded  by  a  border, 
or  narrow,  dense,  opaque  black  hem,  formed  from 
exceedingly  minute  granules  of  an  unknown  iron 
compound  —  the  so-called  "  opacite."  Often  the  OPACITIC- 

BORDERED 

whole  crystal  has  undergone  such  an  igneous  meta-  HORNBLENDE. 
morphosis  and  only  remnants  of  the  fresh,  brown,  original 
mineral  are  to  be  found. 

This  opaque  bounding  of  hornblende  and  biotite  must  not 
be  confounded  with  the  decompositions  effected  by  water, 
whereby  such  a  marginal  hem  is  formed,  proved  to  be  of 
magnetite.  In  this  case  the  hornblende  is  not  perfectly  fresh, 
but  is  partially  changed  to  chlorite,  and  the  opaque  hem  is 
not  so  dense  as  those  crystals  metamorphosed  by  fire. 

Finally,  the  occurrence  of  the  so-called  "  Pseudo-crystals"  of 


QO         DETERMINATION  OF  ROCK-FORMING  MINERALS. 

hornblende,  augite,  and  biotite  must  be  briefly  noticed.  In 
the  younger  eruptive  rocks  bearing  these  minerals,  aggregates 
of  minute  augitic  granules,  feldspathic  grains,  and  especially 
magnetite  or  hematite  leaflets  often  occur,  which  assume  their 
crystalline  forms  ;  often  a  fresh,  irregular,  partially-fused  kernel 
of  hornblende  or  biotite  or  augite  is  seen  within.  It  is  very 
probable  that  these  aggregates  occurring  in  the  eruptive  rocks 
have  been  formed  by  the  action  of  the  liquid  rock-magma  on 
the  unchanged  hornblende,  biotite,  or  augite  crystals,  the  form 
of  the  crystal  being  meanwhile  preserved.  These  pseudo- 
crystals  can  be  formed  experimentally  by  dipping  hornblende, 
etc.,  crystals  in  fused  rock-magma  and  allowing  to  cool. 

The  Shell-formed  Structure  of  Crystals. 

A  macroscopical  examination  of  many  crystals  shows  a 
zonal  structure,  e.g.  barite,  tourmaline,  epidote,  garnet,  etc.;  the 
shell-structure  proves  a  repeated  interrupted  growth  of  the 
crystal,  each  layer  or  coat  corresponding  to  a  period  of  growth. 
This  shell-structure  may  be  easily  shown  in  artificial  crystals  by 
suspending  a  crystal  successively  in  different  mother-liquors ; 
e.g.,  an  octahedron  of  alum  in  a  solution  of  chrome-alum. 

In  the  same  way  an  exceedingly  detailed  laminated  forma- 
tion may  be  observed  often  in  the  microscopical  crystal  individ- 
uals occurring  as  rock-constituents.  Among 
these,  the  feldspars,  augite,  hornblende,  mel- 
anite,  tourmaline,  more  rarely  epidote,  titan- 
ite,  disthene,  andalusite,  corundum,  hauyn, 
nepheline,  etc.,  must  be  mentioned  particu- 
larly. 

The  different  layers  are   often  very  nu- 
merous  and    exceedingly  thin,  and  can   be 
ZONALLY-DEVKLOPED      dist  in  gu  ished     from    each    other    easily,    es- 

AUGITE. 

Section ii oo POO.  pecially  if  multicolored,  as  is  so  commonly 
the  case  with  augite  (Fig.  4$)  or  hornblende,  where  a  green 


METHODS  OF  INVESTIGATION,  9! 

centre  is  surrounded  by  a  brown  layer,  or  green  and  brown  or 
nearly  colorless  layers  alternate.  In  melanite  dark-brown  lay- 
ers alternate  with  lighter;  in  andalusite  often  a  red  centre,  in 
disthene  and  corundum  a  blue  centre,  is  enveloped  by  a  colorless 
coating. 

In  many  cases  the  shell-formed  structure  of  crystals,  as  in  the 
feldspars,  augite,  and  hornblende,  is  first  evident  in  polarized 
light ;  the  different  layers  thus  show  different  polarization-col- 
ors, and  also  the  direction  of  extinction  varies  somewhat  in 
them,  due,  it  appears,  to  the  slight  variation  in  chemical 
constitution  of  the  successive  layers.  These  lines  of  growth 
run  undisturbed  through  the  twinnings  of  the  feldspars,  etc.; 
this  would  indicate  that  the  laminated  development  was 
synchronous  with  the  formation  of  the  twins. 

The  single  layers  often  can  be  distinguished  from  each 
other  more  sharply  by  the  inclosures  of  fluids,  glass,  or  micro- 
lites  lying  between  them  ;  the  successive  layers  have  a  course 
nearly  parallel  with  the  central  crystal  (see  Fig.  45).  Now  and 
then,  however,  crystals  are  observed,  especially  of  feldspar  and 
augite,  where  the  edges  and  angles  of  the  kernel-crystal  are 
replaced  by  faces  of  the  enveloping  layers. 

As  already  mentioned,  a  very  common  and  prominent  de- 
velopment of  crystals  from  two  zones  of  different  optical  orien- 
tation is  noticed  in  the  feldspars,  in  sanidine,  as  well  as  in 
some  species  of  plagioclase.  In  these  latter  species  it  can  be 
proved  often  that  the  kernel-crystal  is  a  plagioclase  of  more  basic 
composition  ;  but  the  envelopes,  on  the  other  hand,  belong  to  a 
plagioclase  richer  in  silicic  acid  and  sodium.  Hoepfner  (N. 
Jahrb.  f.  Min.  u.  Geol.,  1881,  II.  p.  883)  first  called  attention 
to  these  relations  by  showing  that  the  plagioclase  of  andesite 
from  Monte  Tajumbina  often  has  an  anorthite  centre  sur- 
rounded by  an  envelope  of  oligoclase.  Becke  confirmed 
these  observations  on  the  feldspars  in  kersantite  from  the 
lower  Austrian  forest  (Tscher.  Min.  Mitth.,  1882,  V.  p.  161). 


92 


DETERMINATION  OF  ROCK-FORMING  MINERALS. 


The  change  from  kernel  to  envelope  is  quite  gradual,  as  each 
successive  layer  deposits  a  feldspar  richer  in  sodium.  The 
observation  of  Rosenbusch  that  the  decomposition  of  a  feld- 
spar is  generally  from  the  centre  outward  is  quite  in  harmony. 
The  hypothesis  already  proposed  by  the  same  investigator, 
that  the  kernel  of  these  species  of  plagioclase  possesses  a 
more  basic  constitution,  and  therefore  undergoes  an  alteration 
first,  is  confirmed  by  the  observations  of  Hoepfner  and  Becke. 
A  peculiar  structure  of  crystal  is  the  so-called  "  hour-glass 
structure"  as  seen  not  rarely  in  monoclinic  augite  of  many 
basaltic  rocks  (Figs.  46  and  47),  especially  of  limburgite, 


FIG.  46. — AUGITE  WITH  "  HOUR-GLASS 
STRUCTURE."    Section  II  ooPoo. 
(After  L.  v.  Werveke.) 


FIG.  47. — SCHEMATIC  REPRESENTATION 
OF  THE  "HOUR-GLASS  AUGITE." 


more  rarely  in  hornblende,  and  also  in  andalusite  and  stauro- 
lite.  Sections  parallel  to  the  plane  of  symmetry  divide  into 
four  fields  in  polarized  light,  any  two  of  which  lying  opposite 
each  other  show  the  same  colors  and  the  same  optical  orienta- 
tion. The  deviation  in  optical  orientation  is  generally  slight. 
The  sections  parallel  coPoo  are  similar. 

While  sections  perpendicular  to  the  vertical  axis  show  the 


METHODS  OF  INVESTIGATION.  93   ? 

ordinary  zonal  structure.  At  first  a  crystal-skeleton  shaped  li 
an  hour-glass  appears  to  have  been  formed,  both  of  whose 
conical  spaces  were  filled  subsequently  with  an  augitic  sub- 
stance varying  somewhat  in  chemical  composition. 


Interpenetration  of  the  Rock-constituents. 

Graphic  granite  or  pegmatyte  serves  as  one  of  the  best- 
known  examples  of  a  regular  interpenetration  of  two  rock- 
constituents.  In  pegmatyte  numberless  macroscopic  quartz 
individuals  all  showing  the  same  optical  orientation  are  formed 
within  large  orthoclase  individuals.  The  same  penetration 
precisely  is  found  commonly  among  the  microscopic  individu- 
als of  the  rock-constituents,  and  is  called  the  "  micro-pegmatitic 
structure."  This  proves  a  nearly  simultaneous  formation  of 
both  mutually-developed  individuals,  and  occurs  very  com- 
monly in  the  granites  and  crystalline  schists.  In  the  latter 
case,  however,  not  only  is  the  orthoclase  regularly  developed 
with  quartz,  but  also  other  constituents,  as  garnet  or  augite 
with  quartz,  plagioclase  with  augite,  etc.  Their  development 
is  often  irregular,  in  that  the  augite  grains  penetrating  the 
plagioclase  individuals,  e.g.,  do  not  show  throughout  the  same 
optical  orientation.  Regular  interpenetrations  commonly  oc- 
cur also  between  the  augites  and  hornblendes,  where  either 
monoclinic  augite,  especially  diallage  or  omphacite,  also  pos- 
sessing the  brachy-pinacoidal  separation  (Absonderung],  is 
grown  into  the  monoclinic  hornblende  so  that  the  ortho-pin- 
acoidal  faces  of  both  lie  parallel  ;  or  rhombic  and  monoclinic 
augite  are  interpenetrated  so  that  both  lie  with  the  ortho-  or 
macro-pinacoids  adjoining. 

III.    INCLOSURES   OF   THE   ROCK-FORMING   MINERALS. 

Macroscopical  inclosures  have  been  observed  in  many 
crystalline  minerals  for  a  long  period  ;  quartz  is  especially  rich 


94 


DETERMINATION  OF  ROCK-FORMING  MINERALS. 


in  them.  The  microscopic  constituents  of  the  rocks  also  con- 
tain inclosures  many  of  which  may  be  regarded  as  character- 
istic for  certain  minerals.  Among  these  inclosures  of  the  rock- 
components  are  gas-pores,  fluids,  vitreous  particles  (of  the 
rock-mass),  and,  finally,  inclosures  of  other  minerals  also  shar- 
ing in  the  composition  of  the  rock. 


Gas-pores  (Fig.  48). 

During  the  development  of  a  crystal  minute  bubbles  of  air 
often  cling  fast  to  the  faces,  which  afterward  are  surrounded 

and  finally  inclosed  by  the  crystal- 
line material  during  the  succeeding 
growth ;  this  phenomenon  can  be 
best  observed  with  artificial  crystals 
on  removal  from  the  solution.  In  ex- 
actly the  same  manner  bubbles  of  gas 
which  were  absorbed  by  the  mother- 
liquor  and  are  of  such  common  occur- 
rence in  the  vitreous  ground-mass  of 
rocks  were  inclosed  by  the  rock-form- 

GAS-PORES  AND  FLU.D  INCLOSURES.  jng  minerals  during  their  separation 
from  the  molten  magma :  these  are  the  so-called  Gas-pores.  It 
is  difficult  to  determine  what  gases  are  inclosed  within  the 
minute,  generally  egg-shaped  or  irregularly-defined  spaces  ;  it  is 
very  probable  that  gaseous  (i.e.  condensed)  carbon  dioxide  is 
of  common  occurrence.  The  gas-pores  are  often  regularly  dis- 
tributed through  the  crystals ;  being  sometimes  zonal,  parallel 
to  the  crystal  faces  if  they  are  inclosed  between  two  successive 
concentric  layers,  or  forming  an  elongated  series. 

The  minerals  of  the  hauyn  group  among  the  rock-con- 
stituents are  especially  rich  in  inclosures  of  gas-pores  ;  apatite, 
the  feldspars,  augite,  etc.,  also  contain  them.  Cavities  empty, 
or  filled  with  gas,  often  occur,  especially  in  quartz,  which  ex- 


FlG.    48. 


METHODS   OF  INVESTIGATION.  95 

hibit  the  form  of  the  mineral  in  which  they  occur — the  so-called 
"  negative  crystals."  Such  regular  pores  filled  with  air  occur 
in  artificial  crystals  ;  e.g.,  the  cube-shaped  cavities  in  rock-salt. 
During  the  development  of  this  mineral  regular  cubical  de- 
pressions are  formed  ;  an  air-bubble  forces  its  way  into  the 
depression,  which  becomes  covered  afterward  by  succeeding 
depositions  of  the  crystalline  material. 

Fluid  Inclosures  (Fig.  48). 

If  the  mother-liquor  is  forced  into  the  irregular  or  cubical 
cavity  mentioned  in  the  last  example,  instead  of  air  or  other 
gases  absorbed  by  the  mother-liquor,  fluid  inclosures  are  formed 
which  contain  a  small  air-  or  gas-bubble,  sometimes  called  the 
"  libella,"  which  by  turning  the  piece  of  salt  vibrates  along  the 
sides  of  the  cavity. 

In  just  the  same  way  the  fluid  inclosures  commonly  occur- 
ring, especially  in  quartz,  are  formed  in  the  rock-forming  min- 
erals. The  fluid  inclosures  occur  more  rarely  in  the  younger 
and  recent  eruptive  rocks,  and  are  for  the  most  part  inclosures 
of  liquid  carbon  dioxide — a  proof  that  these  rocks  were  formed 
under  immense  pressure.  Inclosures  of  aqueous  solutions  also 
occur  in  the  constituents  of  the  volcanic  rocks ;  it  is  probable 
that  these  liquids  were  inclosed  in  a  fluid  condition.  The 
formation  of  the  bubble  within  the  fluid  inclosures  can  be 
accounted  for  most  easily  by  supposing  that  the  crystals 
separated  at  a  high  temperature  and  under  heavy  pressure  ; 
on  subsequent  cooling  the  inclosed  liquid  contracted  and  thus 
left  an  empty  space — the  bubble.  The  bubble  in  the  micro- 
scopic fluid  inclosures  commonly  shows  a  perfect  freedom  of 
motion,  at  one  time  slow  and  again  exceedingly  rapid. 

If  the  inclosed  liquid  was  a  concentrated  salt  solution, 
minute  crystals  have  been  deposited  since  the  cooling,  and 
liquid,  crystals,  and  bubble  can  be  distinguished  within  the 
cavity.  The  form  of  the  fluid  iriclosure  is  generally  an  irregu- 


<)6         DETERMINATION   OF  ROCK-FORMING  MINERALS, 

lar  one  ;  the  egg-shaped  and  spherical  are  more  rare ;  and  more 
rare  yet  those  assuming  the  form  of  the  inclosing  mineral,  as 
occasionally  in  quartz,  gypsum,  etc.  The  inclosure  is  com- 
monly very  small  and  does  not  generally  exceed  some  hun- 
dredths  or  thousandths  of  a  millimetre.  What  has  been  said 
already  concerning  the  distribution  of  the  gas-pores  is  equally 
applicable  to  these  fluid  inclosures. 

As  regards  the  chemical  constitution  of  the  inclosed  liquids, 
all  determinations  up  to  the  present  time  have  shown  them  to 
be  either  water,  liquid  carbon  dioxide,  or  some  salt  solution,  es- 
pecially of  sodium  chloride.  The  majority  of  the  simply 
aqueous  inclosures  have  a  quiescent  or  feebly-vibrating  bubble, 
which  does  not  disappear  on  heating  to  about  100°  C.  The  in- 
closures of  liquid  carbon  dioxide,  on  the  other  hand,  have  gen- 
erally a  very  mobile  bubble  which  disappears  on  heating  to 
about  32°  C.  If  the  bubble  in  such  an  inclosure  is  very  large, 
i.e.  but  little  liquid  is  present,  the  liquid  CO2  is  changed  into 
the  gaseous  condition  when  the  bubble  disappears ;  if,  on  the 
other  hand,  the  bubble  is  so  minute  that  the  whole  space  is  filled 
through  the  expansion  of  the  liquid  carbon  dioxide,  the  gaseous 
bubble  disappears. 

Inclosures  of  liquids  of  two  kinds  commonly  occur  where 
liquid  carbon  dioxide  is  present  together  with  a  purely  aqueous 
inclosure  in  one  and  the  same  mineral  grain  ;  also,  but  rarely 
in  quartz,  two  different  liquids  are  inclosed  in  one  and  the 
same  cavity,  without  commingling ;  in  this  case  the  inner  liquid, 
generally  carbon  dioxide,  possesses  a  bubble. 

Inclosures  of  concentrated  salt  solutions  have,  for  the  most 
part,  an  immovable  bubble,  or  at  least  one  moving  but  slowly  or 
after  warming,  and  minute  crystals  deposited  from  the  inclosed 
mother-liquor;  sodium-chloride  crystals  are  the  most  common. 
The  bubble,  as  well  as  the  minute  cube,  does  not  disappear  on 
warming  the  preparation,  or  disappears  first  at  higher  tempera- 
tures. 


METHODS  OF  INVESTIGATION. 


97 


The  bubble  is  wanting  in  many  fluid  inclosures,  as  the 
cavities  are  completely  filled  with  liquid.  Such  microscopic 
cavities  can  be  distinguished  from  gas-pores  only  with  great 
difficulty.  They  are  surrounded  in  transmitted  light  with  a 
broad  dark  border,  in  consequence  partly  of  a  total  reflection 
of  the  rays  ;  the  two  can  be  distinguished  only  by  the  presence 
of  the  bubble.  This  dark  border  of  the  gas  and  fluid  inclos- 
ures will  be  the  stronger  the  greater  the  indices  of  refraction  of 
the  inclosed  gas  and  the  inclosing  mineral.  For  this  reason 
the  gas-pores  have  always  a  darker  border  than  the  fluid 
inclosures. 


Inclosures  of  Vitreous  Particles. 

Particles  of  the  ground-mass,  either  purely  vitreous  or  semi- 
individualized,  become  inclosed  during  the  process  of  crystal- 
lization from  the  fused  magma,  just 
as  fluids  are  inclosed  within  crystals 
deposited  from  solution.  These  very 
minute  and  irregular,  egg-shaped,  or 
spherical  glassy  particles,  the  vitreous 
inclosures  (Fig.  49),  solidified  during 
or  after  their  inclosure,  generally 
have  one  or  several  gas-bubbles  in- 
closed with  them.  This  gas-bubble  is 
of  course  immovable,  and,  unlike  the 
bubble  of  the  fluid  inclosures,  cannot 
be  moved  by  heating. 

The  vitreous  inclosures  in  minerals  are  colorless  or  brown 
according  as  the  vitreous  matrix  of  the  rock  is  light  or  dark 
colored  (the  acidic  lavas  have  generally  only  a  colorless,  basic, 
light-colored  or  brown  glass) ;  both  varieties  very  commonly 
occur  together,  the  coloration  of  the  glass  depending  merely 
on  the  amount  of  iron  present. 


FIG.  49.— VITREOUS  INCLOSURES. 


98         DETERMINATION  OF  ROCK-FORMING  MINERALS. 

The  distribution  of  the  vitreous  inclosures  is  either  an 
irregular  one  or  is  in  zones  corresponding  to  the  shell-formed 
structure  of  the  crystal.  Sometimes  the  kernel  of  the  crystal 
is  filled  with  these  inclosures,  and  the  enveloping  layers  poor 
in  them,  or  the  reverse. 

The  vitreous  inclosures  are  especially  common  in  the  feld- 
spars of  the  younger  and  recent  eruptive  rocks,  common  also 
in  quartz  and  augite. 

Vitreous  inclosures  of  dihexahedral  form  are  occasionally 
found  in  quartz,  corresponding  to  its  crystalline  form.  Such 
regular  inclosures  are  formed  in  the  same  manner  as  the 
dihexahedral  gaseous  or  fluid  inclosures  in  quartz,  with  this 
difference,  that  the  substance  filling  the  regular  cavities  is  in 
this  case  a  vitreous  mass.  A  jagged  bubble  is  often  seen  in 
such  vitreous  inclosures,  or  a  gas-bubble  partially  freed  from 
the  inclosure,  which  was  prevented  from  escaping  by  rapid 
deposition  of  crystalline  matter.  The  presence  of  such  an 
escaping  bubble,  as  well  as  that  of  several  bubbles  within  the 
inclosures,  is  a  proof  of  their  solid  vitreous  character:  such 
phenomena  could  not  occur  in  fluid  inclosures. 

Minute  crystals,  magnetite  octahedra,  augite  microlites, 
trichites,  etc.,  have  often  separated  during  solidification  of  vit- 
reous, inclosed  particles  in  the  same  manner  as  crystals  are  de- 
posited from  inclosures  of  saturated  solutions ;  i.e.,  the  glass  is 
"devitrified"  (entglasf).  The  magnitude  of  the  gas-bubble 
has  absolutely  no  genetic  connection  with  the  magnitude  of 
the  inclosure.  The  vitreous  inclosures  will  show  in  transmit- 
ted light  no  such  dark  border  as  the  gas-pores  and  fluid  inclos- 
ures, as  the  index  of  refraction  of  the  glass  is  rather  high  and 
differs  less  than  air  or  water  from  that  of  the  mineral.  The 
vitreous  portion  of  the  inclosure  has  consequently  a  less 
marked  border,  although  the  gas-bubble  shows  all  the  darker 
broad  band. 

The  presence  of  a  gas-bubble  in  the  vitreous  inclosure  cut 


METHODS  OF  INVESTIGATION.  99 

through  in  the  process  of  grinding  the  mineral  section  is  an 
additional  means  of  discriminating  between  a  vitreous  and  a 
fluid  inclosure.  As  the  gas-bubble  is  an  empty  cavity,  fixed  in 
the  solid  vitreous  body,  it  is  cut  through  during  the  process  of 
preparation,  becomes  filled  with  Canada  balsam,  and  the  vit- 
reous inclosure  appears  in  the  preparation  as  only  a  feebly  out- 
lined circle;  a  fluid  inclosure,  on  the  other  hand,  thus  cut 
through  would  become  completely  filled  with  Canada  balsam, 
as  the  liquid  escapes  during  the  process  of  cutting  and  the  bub- 
ble in  this  case  is  completely  dissipated. 

Often  large,  irregular  particles  of  non-  or  but  poorly-indi- 
vidualized vitreous  masses  with  no  inclosed  gas-bubbles  occur 
in  the  rock-forming  minerals  ;  as,  e.g.,  between  the  layers  or  in 
the  kernel  of  feldspar,  olivine,  etc.  They,  as  well  as  the  vitreous 
inclosures  containing  gas-bubbles,  are  a  proof  of  the  formation 
of  the  rock  (i.e.  the  minerals)  from  a  molten  magma. 

In  the  quartz  grains  of  rock  of  undoubted  sedimentary  ori- 
gin which  were  solidified  from  confined  and  metamorphosed 
eruptive  rocks,  vitreous  inclosures  are  also  discovered,  but  of  a 
secondary  character,  being  first  formed  through  the  action 
of  eruptive  magma  heated  to  redness -on  the  inclosed  rock;  this 
can  be  proved  by  experiment.  The  way,  however,  in  which 
such  secondary  vitreous  inclosures  could  be  made  is  at  present 
unexplained.  (See  Chrustschoff,  Tschermak's  Min.  Mitth. 
1882,  IV.  p.  473.) 


Inclosures  of  Foreign  Minerals. 

Macroscopical  inclosures  of  other  minerals  have  been  ob- 
served commonly  in  quartz  (prase,  etc.).  Among  the  micro- 
scopical constituents  also,  quartz,  as  well  as  many  other  min- 
erals, as  staurolite,  etc.,  is  especially  rich  in  inclosures.  The 
granules  or  crystals  thus  inclosed  within  the  rock-constituents 


IOO      DETERMINATION  OF  ROCK-FORMING  MINERALS. 


are  of  those  minerals  making  up  the  composition  of  the  par- 
ticular rock,  and  are  for  the  most  part  very  minute  and  often 
regularly  distributed  through  the  inclosing  mineral.  In  augite, 
e.g.,  long,  narrow  indeterminable  microlites  (augite  ?)  together 
with  vitreous  inclosures  are  commonly  arranged  in  zones  ; 
these  were  inclosed  in  the  same  manner  as  the  vitreous  parti- 
cles, during  separation  of  the  crystal  from  the  vitreous,  semi- 
individualized  magma.  In  other  minerals  the  mineral  inclos- 
ures are  regularly  distributed  parallel  to  cer- 
tain faces,  as  the  opaque  to  brownish  trans- 


lucent rectangular  tablets  parallel  to 
in  hypersthene  and  bronzite  (Fig.  50),  or  the 
opaque  microlites  and  tablets  parallel  to  the 
r'-axis  in  labradorite. 

The  zonally  arranged  inclosures  of  small 
quartz  granules  in  the  garnet  and  staurolite 
INCLOSURKS  o?  BROOK-    of  the   crystalline  schists;  the  inclosures  of 

ITE  (?)  TABLETS  IN 

HYPERSTHENE.  minute  elongated  needles  of  rutile,  regular 
and  crossed  at  an  angle  of  60°,  occurring  in  some  species  of 
magnesian  micas  in  certain  eruptive  rocks  ;  and,  finally,  the  in- 
closures of  sillimanite  microlites  in  cordierite  and  quartz  of 
crystalline  schists,  etc.,  are  also  especially  worthy  of  mention. 

Comparison  of  the  inclosures  of  rock-constituents  often 
proves  of  importance  in  determining  the  order  of  separation,  i.e. 
the  formation  ;  thus  magnetite,  menaccanite,  spinel,  rutile,  zir- 
conite,  are  generally  the  minerals  first  formed  in  the  crystalline 
rocks,  as  they  are  always  found  included  within  all  the  min- 
erals occurring  in  one  and  the  same  rock. 

In  the  eruptive  rocks  the  magnesian  silicates  generally 
followed  these  in  order  of  separation  (augite,  hornblende,  bio- 
tite,  and  olivine),  then  the  feldspars,  and  finally  quartz.  Never- 
theless no  universal  law  can  be  formulated.  Still  less  possible 
is  it  to  formulate  a  law  for  the  crystalline  schists.  Quartz,  and 
also  orthoclase,  are  found  included  within  hornblende  and  gar- 


METHODS   OF  INVESTIGATION.  IOI 

net — i.e.,  they  were  first  formed  ;  or  quartz  and  orthoclase  are 
interpenetrated  (micro-pegmatitic,  graphic-granitic) — i.e.,  both 
were  developed  at  the  same  time. 

The  chemical  and  physical  properties  of  minerals  are  of 
course  changed  by  these  inclosures.  Specimens  as  free  as 
possible  from  inclosures  must  therefore  be  selected  for  ex- 
amination. 


IV.  DECOMPOSITION  OF  THE  ROCK-CONSTITUENTS. 

J.  ROTH.     Allgemeine  und  chemische  Geologic.     Berlin,  1879.  I.  Bd. 

The  rock-forming  minerals  are  far  more  exposed  to  the 
decomposing  and  solvent  influences  of  filtrating  waters  than 
the  larger  developed  minerals.  In  the  volcanic  rocks  a  further 
change  of  the  rock-constituents  is  induced  by  the  action  of 
the  gaseous  emanations  accompanying  the  eruptions.  For 
these  reasons,  therefore,  different  minerals  are  found  in  the 
rock-preparations  in  different  stages  of  decomposition.  The 
metamorphosis  in  mo|t  cases  can  be  studied  and  followed  on 
the  thin  sections.  It  begins  almost  always  from  without  and 
advances  inwards,  especially  on  the  cleavage-fissures  of  crystals 
or  grains ;  the  crystal-kernel,  as  in  the  feldspars,  though  rarely, 
first  undergoes  decomposition. 

Olivine,  orthoclase,  and  magnetite,  of  the  rock-forming 
minerals,  most  commonly  occur  thus  metamorphosed. 

In  the  metamorphosis  of  olivine  into  serpentine,  fine  green- 
ish or  brown  thread-like  aggregates  appear  along  the  fissures. 
These  gradually  broaden,  whereby  the  cross-section  of  olivine 
on  the  slide  seems  drawn  into  a  net  of  serpentine,  in  whose 
meshes  lie  the  fresh  olivine  residues.  These  also  finally  under- 
go decomposition,  and  a  complete  pseudomorphosis  of  serpen- 
tine after  olivine  results. 

Serpentine  is  generally  tinged  red  by  freshly-formed  iron 


IO2       DETERMINATION  OF  ROCK-FORMING  MINERALS. 

hydroxide.  Clino-chlore  is  deposited  in  many  cases  in  olivine- 
fels  by  the  metamorphosis  of  magnetite.  In  these  cases  water 
is  taken  up  and  magnetite  and  iron  silicates  are  deposited. 
If  the  silicates  are  removed  so  that  only  the  ferrous  oxide 
separated  from  olivine  remains  as  ferric  oxide  and  hydroxide, 
pure  pseudomorphs  of  ferric  oxide  and  hydroxide  after  olivine 
are  often  formed. 

Grayish  to  brownish  opaque  pseudomorphoses  after  olivine 
are  often  found  in  the  picrites,  consisting  principally  of  calcite 
and  showing  a  mesh-like  structure.  The  meshes  themselves 
are  formed  from  calcium  silicate,  while  the  spaces  between 
are  filled  with  calcite.  In  this  case  silicic  acid  and  magnesia 
are  removed,  and  alumina,  lime,  carbon  dioxide,  and  alkalies 
are  taken  up.  Similar  pseudomorphs  of  calcite  after  augite 
also  occur. 

In  the  metamorphosis  of  feldspar  into  kaolin  no  such  regu- 
lar progress  of  decomposition  beginning  with  the  cleavage- 
fissures,  as  a  rule,  can  be  determined  ;  they  become  spotted 
and  opaque,  and  metamorphosed  into  an  aggregate  of  minute 
gray  or  white  grains.  The  alumina  remains  constant,  silicic 
acid  is  partially  removed,  water  and1  potassium  are  taken  up. 
In  the  zonally-developed  feldspars  the  layers  rich  in  inclosures 
first  undergo  decomposition. 

Potassium  micas,  in  minute  brilliantly-polarizing  tablets, 
are  also  commonly  formed  by  the  decomposition  of  the  feld- 
spars ;  quite  perfect  pseudomorphs  of  muscovite,  after  ortho- 
clase,  are  often  found.  In  this  case  the  greater  part  of  the 
alkali  remains;  the  rest  is  removed  together  with  silicic  acid, 
which  often  separates  as  quartz  (SiO2). 

Menaccanite  becomes  coated  with  a  gray  opaque  coating 
(leucoxene),  and  is  finally  metamorphosed  into  transparent 
titanite  ;  lime  must  be  added.  More  rarely  menaccanite  meta- 
morphoses into  rutile  with  separation  of  ferric  oxide,  which  de- 
posits as  a  reddish  border  about  the  decomposed  mineral. 


METHODS  OF  INVESTIGATION.  1 03 

Finally,  mention  must  be  made  of  the  metamorphosis  of 
minerals  of  the  hauyn  group,  and  of  nepheline  into  the  zeolites, 
especially  natrolite,  wherein  calcite  often  separates ;  the  meta- 
morphosis of  garnet  into  chlorite  ;  of  biotite,  hornblende,  and 
augite  into  chlorite  and  epidote,  with  elimination  of  quartz, 
ferric  hydroxide,  and  calcite  ;  the  decomposition  of  rhombic 
augite  in  bastite,  etc. 


END   OF  PART  I. 


PART    II. 

TABLES  FOR  DETERMINING  MINERALS. 


ABBREVIATIONS   USED   IN  THE  TABLES. 

Under  the  heading  "  Optical  Properties:" 

AP  —  Plane  of  the  optic  axes. 

1  M.  =  First  middle  line. 

2  M.  =  Second  middle  line. 

a  =  Axis  of  greatest  elasticity, 
b  =  Axis  of  middle  elasticity  —  optic  normal, 
c  =  Axis  of  least  elasticity. 
I  =  Parallel. 
_L  =  At  right  angles. 
n  =  Index  of  refraction. 

For  optically-     j  GO  =  Index  of  refraction  for  the  ordinary  ray. 
uniaxial  minerals.  (    £  =  Index  of  refraction  for  the  extraordinary  ray. 
For  optically-      I  ft  =  Index  of  refraction  of  middle  value, 
biaxial  minerals.  (  p  =  For  red  light. 

i.  c.  p.  1.  =  In  convergent  polarized  light, 
i.  p.  p.  1.  =  In  parallel  polarized  light. 
For  the  crystallographic  axes  : 

c  =  Chief,  i.e.  vertical,  axis. 
In  rhombic  or      i  a  =  Brachydiagonal  axis, 
triclinic  minerals.  (  ~b  =  Macrodiagonal  axis. 
In  monoclinic  i  a  =  Clinodiagonal  axis, 
minerals.       (  V  =  Orthodiagonal  axis. 
Under  the  heading  "Structure:" 

I.  O.  =  Components  first  in  order  of  separation. 
II.  O.  =  Components  second  in  order  of  separation. 


io6 


DETERMINATION  OF  ROCK-FORMING  MINERALS. 


hO 


U  CD 


0) 

4-> 

ho 


Q 


rt 
H 


o-l 
«  §  > 

HTI^ 
O  K  <u 

SH 

I1 


*GS 


al. 

ho 
co 
g 


Optically- 
Isotropic  sec 
a  fixed  axial  c 
verging  pol 


—   CO 

-CB  rt 


one 
ish 
ral 


el 
nd, 
ique 


om  the  zo 
extinguis 
tions  para 
e  other  ha 
nction  obl 
oP  '  :  ooPo 
o  ;  ooPoo. 


m 


1 


iU 

liilg 


~   05  u  3 

<  e 


rt   0   <u 

I1 


in 


TABLES  FOR  DETERMINING  MINERALS. 


optic  axes  show  in  con- 
without colored  rings, 
ccording  as  the  section 
,  the  axial  point  appears 
tage  this  cloud  revolves 
yperbola,  is  red  on  the 
e  dispersion  of  the  axes 
clination  of  the  axes  of 
tions  parallel  oojPoo  in 
v>  in  the  triclinic,  gives 


sis  alf 

B:Y*«3'B«3?v,fl    §• 


angles  to  one  of  the 
black  cloud  with  or 
double-refraction. 
lar  to  the  middle  line, 
.  On  revolving  the  st 
If  this  cloud,  i.e.,  hy 
the  concave,  then  th 
>  p.  The  angle  of  in 
ographic  axes  in  sect 
parallel  oP  and  <x>P 
termining  them. 


c  sections 
polarize 
g  to  the 
or  less  pe 
t  or  within 
ntrary  dir 
side  and 
v\  if  the  re 
ity  to  the 
linic  miner 
rable  me 


Isot 
ver 
acco 
is  m 
wit 
in  a 
con 
is  p 
e^ast 
mon 
an  a 


,o7 


Transverse  sections 
show  a  fixed  black 
axial  cross,  with 
or  without  colored 


gs,  according  to  th 
power  of  double- 
refraction  and  the 
thickness  of  the 
mineral  leaflet. 


Jis!  si 

rt 


—  0 


- 

<       «    8    8  *       rt 


sijSuv 


pazuBpj  SinjfcdAuoQ  m  uoi;Burarexg[ 


io8 


DETERMINATION  OF  ROCK-FORMING  MINERALS. 


A.  Even  in  the  thinnest  Sections 


NAME. 

Chemical 
composition 
and  reactions. 

Specific 
gravity. 

System 
of 
crystalliza- 
tion. 

Cleavage. 

Ordinary 
combinations 
and  form, 
of  the 
cross-section. 

Twins. 

1.  Magnetite. 

(Magnet- 

Fe304 
(FeO  +  Fe203). 

4.9-5.2. 

According 
toO. 

Grains  and 
octahedra. 

According 
toO. 

eisen.) 

Easily  soluble 
in  HC1. 

. 

Squares  and 
equilateral 

^ 

triangles. 

t 

\ 

2.  Titaniferous 

FeO 

4.8-5.1. 

* 

Octahedra 

Magnetite. 

**  e*-'   1    KeTiOq 

and 

(  Titan- 
magneteisen.) 

Distinguished 
from 
magnetite  only 
by  chemical 

£ 

§ 
6 

K 

grains. 

analysis. 

(Reaction  for 

titanium.) 

3.  Pyrite. 

FeS3. 
Easily  soluble 
in  HN03, 
with 
separation  of  S. 

4-9-5-2. 

According 

to  oo  O  oo. 

2 

Regular 
hexagons  and 
pentagons. 

Penetration- 
twins  of 

2 

TABLES  FOR  DETERMINING  MINERALS. 

of  Opaque  Minerals. 


109 


Color  and 
lustre. 

Structure. 

Association. 

Decomposition. 

Occurrence. 

Remarks. 

Iron-black; 
in  reflected 

Often  in 
beautiful 

With  nearly 
all  of  the 

Commonly 
into  iron 

i.  As  primary 
essential 

light  bluish- 
black  metallic 
lustre. 

cruciform 
aggregates  ; 
o.-  as  product 
of  decomposi- 
tion wreathed 
about  the 
minerals; 
also  deposited 
upon  the 

rock-forming 
minerals; 
especially  with 
augite, 
olivine, 
plagioclase, 
nepheline, 
and  leucite. 

hydroxide. 
A  reddish- 
brown  circle 
about  the     ( 
magnetite 
crystals. 

*** 

constituent  of  the 
basic  eruptive 
rocks;  accessory 
in  nearly  a"H  of 
P  the  crystallinjBr  — 
^^jpeks. 
•^Tisaecomposi- 
tion-product  of 
olivine,  augite, 

cleavage- 
fissures. 

hornblende, 
and  biotite. 

Ditto. 

iflQB  titanite, 
leucoxene, 
and 
iron  hydroxide. 

Primary;  in 
basaltic  rocks  and 
crystalline  schists. 

Forms  at  the 
same  time 
the  transition- 
products  to 
ilmenite. 

In  reflected 
light 
brass-yellow. 
Metallic 
lustre. 

• 

Into  iron 
hydroxide. 

Rarely  as 
accessory 
secondary 
constituents  in 
decomposed  basic 
eruptive  rocks, 
and  (also  primary) 
in  ciystalline 
schists. 

£ 

110 


DETERMINATION  OF  ROCK-FORMING  MINERALS. 


NAME. 

Chemical 
composition 
and  reactions. 

Sueciflc 
gravity. 

System 
of 
crystalliza- 
tion. 

Cleavage. 

Ordinary 
combinations 
and  form 
of  the 
cross-section. 

Twins. 

4.  Ilmenite. 

FeTi03  + 

4.56-5.21. 

R  and  oR\ 

Tabular 

With  parallel 

(Titan-eisen  ) 

X  (Fe203,. 
Difficultly 
soluble  in  HC1. 

\« 

conchoidal 
separation 
(absonde- 

R  .  oR; 
also 

-  y^R,  -  SA-, 

axial 
systems. 
Polysynthetic 

Ti-reaction 

rung). 

and  grains 

twins 

with 

which  are  not 

af.er  R. 

microcosmic 

spherical 

salt. 

but  for  the 

most  part 

long  rods. 

Cross-sections 

generally 

hexagonal, 

long. 

th  readlike, 

jagged,  or 

netted  forms. 

„• 

rt 

5.  Graphite 

C. 

1.9-2.3. 

rt 

M 

oP. 

Rarely  in  thin 

(and 
bitumen). 

Bituminous 
black  rocks, 

<U 

K 

hexagonal 
tablets  and 

becoming 
grayish-white 
on  heating. 

irregular 
leaves. 

6.  Pyrrhotite. 
(Magnet  kies.) 

FenSn+l. 

4.54-4.64. 

Irregular 
grains. 

MINERALS  RENDERED   TRANSPARENT 


1.  Chromite. 

See  page  14. 

Regular. 

1 

Grains  and 
octahedra. 

2.  Pleonaste. 

3.  Haematite. 

See  page  32. 

Hexagonal. 

Tablets. 

TABLES  FOR  DETERMINING  MINERALS. 


Ill 


Color  and 
lustre. 

Structure. 

Association. 

Decomposition. 

Occurrence. 

Remarks. 

Black-brown; 
•metallic 
lustre. 
In  reflected 

With 
plagioclase, 
augite, 
hornblende, 

Into  titan  ite 
(leucoxene)  and 
rutile  ivitli 
heematite.   Ilmenite 

In  basic 
eruptive  rocks 
(especially  the 
granular 

Can  be 
distinguished 
from  magnetite 
by  the  form 

light  gray, 
if  decomposed. 

and 
olivine. 

is  metamorphosed 
by  decomposition 

diabases, 
gabbros, 

of  the 
cross-section. 

first  into  a 

basalts, 

and  especially 

grayish,  opaque, 
pulverulent  mine- 
ral (leucoxene). 
changing  gradually 

picrites); 
also  in 
crystalline 
schists. 

by  the. 
phenomena 
attending 
decomposition. 

into  one  brown  and 

transparent,  which 

can  be  determined 

as  titanite;  often 

thin  decomposed 

threads  of 

ilmenite  remain. 

(Comp.  Fig.  51.) 

Iron-black  ; 

Rare,  in 

Distinguished 

metallic 

crystalline 

from  haematite 

lustre. 

schists,  clay 

by  its  opacity 

and  clay-mica 

or  decoloration 

schists,  gneiss, 

by  heating. 

limestone,  and 

as  an  inclosure 

in  staurolite, 

andalusite, 

chiastolite, 

dipyre,  and 

couzeranite. 

Bronze-yellow 
and 
copper-red. 

Rarely  in 
crystalline 
schists, 
contact-schists. 

Can  be 

distinguished 
from   pyrite 
easily  by  the 

lustre  in 

reflected  light. 

ONLY  WITH   DIFFICULTY. 


Black; 

metallic  lustre; 

if  transparent, 
brown. 

Rare  in  olivine 

rocks. 

Similarity 

Green. 

Contact  rocks 

with 

and 

magnetite. 

schistose  rocks. 

Red. 

Generally  as 

Similar  to 

accessory 

graphite,  etc. 

constituent  and 

product  of 

decomposition. 

112 


DETERMINATION  OF  ROCK-FORMING  MINERALS. 


B.  Minerals  Transparent 
I.  SINGLE-REFRACTING  MINERALS. 

a.  Amorphous 


Chemical 

NAME. 

composition  and 

Specific  gravity. 

Color. 

Structure. 

reactions. 

1.  Opal, 

(Porodinamorph.) 

Essentially  SiO2 
(H2O.  traces  of 
Fe,  Ca,  Al,  Mg, 

1.9-2.3- 

Colorless 
(white,  yellowish), 
often  colored 

a.  Homogeneous 
and  devoid 
of  structure. 

and  the  alkaloids). 
Soluble  in 
KOH. 

red  and  brown 
by  ferric  oxide 
and  hydroxide. 

b.  Concentric- 
conchoidal,  and 
then  showing 

n  =  1.455. 

often  in 

parallel  polarized 
light  feeble 

double-refraction 

(interference- 

cross). 

In  clusters, 

crust-like. 

V 

(See  Fig.  52.) 

2.  Hyalite. 
(Glasmasse.) 

Always  a 
complicated 
silicate 
(Si,  Al,  Fe,  Ca, 
Mg,  alkalies). 
Acid  vitreous 
mass  containing 
about  70  per  cent 
SiOa  insoluble  in 
HC1. 

Acidic 
=  2.2-2.4 
(obsidian). 
Basic 
—  2.51 
(trachylyte). 

Colorless, 
or  colored  gray, 
brown,  or  red. 
The  basic  hyalite 
is  generally  dark, 
the  acidic  light. 
n  (for  obsidian) 
=  1.488. 

a.  Obsidians 
absolutely  devoid 
of  structure, 
with  separation  of 
free  glasses. 
b.  Pitchstone, 
with  macroscopic 
separations. 
c.  Pumice-stone, 
fibrous  and  filled 

Basic  vitreous 

with  gas-pores. 

mass  with 

d.  Perlite, 

about  40  per  cent 
SiOa,  for  the  most 

spherical  with 
concentric- 

part  soluble  in 

conchoidal 

HC1. 

structure. 

TABLES  FOR  DETERMINING  MINERALS. 

in  Thin  Sections. 

(Isotropic  in  all  cross-sections.) 

Minerals. 


Inclosures. 


Decomposition. 


Occurrence. 


Remarks. 


a.  Brownish -red, 
dust-like  inclosures  of 

ferric  hydrate. 

b.  Aggregates  of 
hexagonal  tablets  of 

tridymite. 

c.  Fluid  inclosures 

and  gas-pores. 


Always  secondary ; 

decomposition- 
product  of  the  rock- 
constituents 
feldspar,  augite, 
hornblende,  biotite, 
and  then  deposited 
either  in  the  primary 

position,  i.e.,  as 
pseudomorphs  after 

these  minerals, 

or  in  some  secondary 

position  lining  the 

walls  of  cavities ; 

especially  in  the 

acidic  younger 

eruptive  rocks,  the 

rhyolites  and 

andesites,  but  also  in 

the  basic  basalts. 


The  ground-mass  of 

many  decomposed 

eruptive  rocks  is 

almost  completely 

metamorphosed  into 

opal  (semi-opal). 


Inclosures,  i.e., 
separations,  are 

gas-fores, 

crystallites,  and 

micro! iths  very 

commonly  ;  also 

crystals  and 

sphaeroliths. 

(Compare  Figs.  41 

and  42.) 


Into  viridite  in  the  basic 
rocks,  basalt  ;  into 
opal  in  the  acidic, 

rhyolite. 

Basic  glasses  are  often 
decomposed  into  a 

yellowish, 

dov.ble-refracting, 

fibrous  substance 

(pelagonite). 


The  natural  glasses 

are  only  one  method 

of  solidification  of  the 

eruptive  rocks. 

Hyalite  occurs  more 

or  less  commonly  in 

often  apparently 

purely  crystalline 

eruptive  rocks,  and 

only  in  them. 


Rock  glasses 

(vitrophyre)  are  known 

in  the  following 

eruptive  rocks: 

a.  Acidic  =  vitreous 
rhyolith,  trachyte, 

dacite,  andesite, 

porphyries;  rarely 

porphyrites  and 

phonoliths. 

b.  Basic  =  vitreous 
diabase,  melaphyr, 

augite-andesite,  and 
basalts  (trachylyte, 
hyalomelane,  sidero- 
melane,  palagonite, 

hydrotachylyte). 

Frequently  pure  hyalite 

can  be  distinguished 

from  opal  only  with 

difficulty;  the  only 

surety  lies  in  the 

micro-ch  em  ica  I 

analysis  (preferably 

corrosion  with  hydro- 

fluosilicic  acid).    The 

glasses  are  mentioned 

here  only  because  of 

their  differences 

from  opal. 


DETERMINATION  OF  ROCK-FORMING  MINERALS. 


b.     Minerals  Crystallising 


Ordinary 

NAME. 

Chemical 
composition 
and  reactions. 

Specific 
gravity. 

Cleavage. 

combinations 
and  form 
of  the 
cross-section. 

Twins. 

Color  and  power 
of 
refracting  light. 

1.  Hauyn 

Group. 

a.  Sodalite. 

3(Na2Al2Si2O8) 
+  2  NaCl. 
Cl-reaction\ 

2.13-2.29. 

oo  0. 

Grains  and 
oo  O  (rarely 

O.ooOoo); 

Penetra- 
tion-twins 
after  a 

Colorless; 
colored  by 
FctO, 

easily  soluble 
in  HC1  ; 
gelatinous 

cross-sections 
rectangular 
and  hexagonal. 

trigonal 
secondary 
axis. 

red,  green, 
and  blue  — 
mostly  blue. 

Si02; 

NaCl-cubes  on 

evaporation. 

• 

0.  Hauyn 
and 
V.  Nosean. 

2Na2CaAl2 
Si208  + 
Na2(Ca)S04. 
Reaction  for 
Ca  and  H2SO4. 

3Na2AI2Si208 

2.4-2.5. 
2.279-2.399 

00  O. 

Crystals 
oo  O  and  O, 
like  sodalite. 

Commonly 
distorted  or 

Twins 
after  O 
and  like 
sodalite. 

Colorless^  blue 
or  black. 

Colorless^ 
broivn  or 

Reaction  for 

corroded 

black. 

H2S04. 

cryrtals. 

Both  soluble 

in  HC1  with 

separation  of 

gelatinous 

SiO2. 

• 

TABLES  FOR  DETERMINING  MINERALS. 

in  the  Regular  System. 


Structure. 

Association. 

Inclosures. 

Decomposition. 

Occurrence. 

Remarks. 

Interpenetrated 
with  feldspar 
and  horn- 

With micro- 
cline,  augite, 
and  mica  in 

Fluid 
inclosures  and 
gas-pores. 

Becomes 
opaque  by 
decomposition 

As  primary 
constituent  in 
syenites 

blende;  the 

syenites. 

into  zeolites. 

(elaeolith- 

centre  often 

Vitreous 

syenite),  and 

colorless,  and 

With  sanidine 

inclosures  and 

rarely  in 

outer  layers 

and  augite 

augite  needles. 

augitic 

blue  or 

in  trachytes. 

trachytes. 

colored  red  by 

ferric  oxide. 

Secondary  in 

The  three  minerals 

cavities  of  the 
latter. 

can  be  accurately 
distinguished 

only  by  the 

tn  icro-ch  em  ical 

qualitative 

analysis. 

The  outer 
coats  colored. 

Generally  with 
leucite, 

Numberless 
gas  pores 

Into  a  felt-like 
aggregate  of 

Primary 
constituent. 

Hauyn  is 
distinguished  from 
sodalite  by 
presence  of  the 

with  opaque 

nefiheline^ 

and  vitreous 

colorless. 

characteristic 

or  dark  core; 
often  colored 
red  by 

and  augite. 

inclosures 
arranged  in 
streamers. 

double- 
refracting 
needles  and 

In  the  younger 
eruptive  rocks 
and  the 

gypsum  needles 
on  evaporating  a 
drop  of  the 

iron  oxide 

Black 

filaments  of 

sanidine  and 

hydrochloric  acid 

at  the  cleavage- 
fissures. 

minute  grains 
and  needles, 

zeolites  and 
calcite.     A 

plagioclase 
rocks,  as 

solution  (on 
account  of  the 

Regularly 

like  dust. 

decoloration  of 

trachyte 

calcium  present 

disposed 
inclosures, 

often  at  regular 
spaces. 

the  hauyn 
thus  occurs  ; 

(rarely), 
phonolite. 

in  hauyn); 
sodalite  is 

and  systems  of 
dark  streaks 

Pyrite  tablets. 

a  yellowish, 
secondary 

leucitophyr, 
tephntes, 

characterized  by 
the  chlorine. 

at  right  angles 

coloration 

nepheline 

to  each  other. 

of  the 

and  leucite 

(See  Fig.  53.) 

decomposition- 

basalts. 

product  by 

*        f            .            J 

ferric 

Very  common 

It  is  difficult  also 

hydroxide. 

in  the 

to  distinguish 

trachytic 

hauyn  from  nosean 

volcanic  lavas. 

chemically. 
Mineralogically 

they  are  one. 

u6 


DETERMINATION  OF  ROCK-FORMING  MINERALS. 


NAME. 

Chemical 
composition 
and  reactions. 

Specific 
gravity. 

Cleavage. 

Ordinary 
combina- 
tions and 
form  of  the 
cross-section. 

Twins. 

Color  and 
power  of 
refracting  light. 

2.  Garnet 
Group, 
a.  Almandine. 

Fe3AlaSi3O13. 

3.78 
(3-1-4.2). 

Imperfect 
oo  O. 

coO.202 
and  grains. 
Cross- 
sections 
quadratic, 
hexagonal, 
or 
octagonal. 

Red,  in  very 
thin  sections 
nearly 
colorless. 
ftp  =  1.772. 

/3.  Pyrope. 

(CaO,  MgO, 
FeO,  MnO) 
Al2033Si02. 
Contains  Cr. 

3.7-38. 

Imperfect 

00  0. 

Grains. 

Blood-red. 

y.  Melanite. 

Ca3Fe3Si3012. 

3.6-4.3. 

Imperfect 
oo  O. 

Crystals 

00  O. 

Black; 
dark  brown 
in  sections. 
Transparent 
only  with 
difficulty. 

All  garnets  are 
insoluble  in  acids. 

TABLES  FOR  DETERMINING  MINERALS. 


117 


Structure. 

Association. 

Inclosures. 

Decomposition. 

Occurrence. 

Remarks. 

Cotntnonly 
disseminated 

Generally  with 
quartz, 

Cavities  of 

the  form  of 

Commonly 
metamorphosed 

Primary 

constituent  ; 

through 
micro-pegmatic 
quartz  and 
feldspar. 

orthoclase, 
biotite,  and 
hornblende. 

garnet 
(=  negative 
crystals). 
Fluid 
inclosures, 

into  chlorite 
tablets  on  the 
upper  surfaces 
and  cleavage- 
fissures.     More 

in  many 
crystalline 
schists, 
common  in 
granite,  rare 

quartz- 

rarely,  as  in 

in  trachytic 

granules. 

pyrope, 

rocks. 

Rutile;  often 

metamorphosed 

zonally 

about  the 

disposed. 

edges  into  a 

(See  Fig.  54.) 

fibrous 

hornblende. 

or  augite-zone. 

The  garnets 
can  be  easily 

distinguished 

from  the 

hauyn  by  the 

color  and 

With  olivine 
and  augite. 

Very  poor. 

Augitic  fibrous 
tufts  shooting 

Primary 
constituent. 

insolubility 
in  acids. 

out  in  marginal 

In  serpentines. 

zones, 

perpendicular  to 
the  surface  of 

the  grains 

are  very  common 

and 

characteristic. 

(See  Fig.  55.) 

Very 
commonly 
beautiful 
zonal 

With  augite, 
sanidine, 
nepheline, 
hauyn,  and 

Very  poor. 
Augite  and 
apatite 
needles; 

Primary 
constituent. 
In  phonoliths, 
leucitophyr. 

Compare  with 
chromite. 

structure, 

leucite. 

vitreous 

and  volcanic 

then 

inclosures. 

lavas. 

generally 

showing1 

double 

refraction. 

(See  Fig.  54.) 

DETERMINATION  OF  ROCK-FORMING  MINERALS. 


Ordinary 

NAME. 

Chemical 
composition 
and  reactions. 

Specific 
gravity. 

Cleavage. 

combinations 
and  form 
of  the 
cross-section. 

Twins. 

Color  and 
power  of 
refracting  light. 

3.  Spinel  Group. 

a.  Chromite. 

FeO,Cr203. 

4.4-4.6. 

Imperfect 

Grains  and 
octahedra. 

Becomes 
translucent 

only  -with, 
difficulty  ; 

dark  bro-wn^ 

reddish  brown, 
metallic 

lustre. 

/3.  Picotite. 

MgO)  A12O3  I 
FeO  j  Fe203  j 

4.08. 

Octahedra. 
Very  minute 

Twins 
according 

ditto. 

grains. 

to  O. 

•y.  Pleonaste. 

FeO  1  A1203  ) 
MgO  f  Fe203  f 

Above  3.65. 
(3.8-4.1). 

ditto. 

Dark  green. 

8.  Hercynite. 

FeO,  A12O3. 

3-9I-3-95. 

Octahedra. 

ditto. 

Insoluble  in 

acids; 

unattacked 

by  HF1. 

4.  Analcime, 

Soluble  in  HC1 

2.1-2.28. 

(Imperfect); 
according 
to 

Generally 
compact  grains: 
in  cavities 

Colorless^ 
white. 
«p  =  1.4874. 

with 

Tschermak, 

202. 

separation 
of  gelatinous 

clearly 
ooOoo. 

SiOa. 

TABLES  FOR  DETERMINING  MINERALS. 


Structure. 

Association. 

Inclosures. 

Decomposi- 
tion. 

Occurrence. 

Remarks. 

Many 

With  olivine 

Great  similarity 

individual 

and  augite. 

Primary 

with  melanite;  if 

grains 
occurring  in 
basalts  show 

accessory 
constituent. 
In  olivine  rocks, 

in  grains,  can 
be  distinguished 
only  by  chemical 

a  broad, 

serpentines, 

tests.     Melanite  is 

opaque  border. 

and  in  basalts. 
Picotite 

attacked  by 
concentrated  HF1, 

commonly 

and  is  free  from 

inclosed  in 

chromium. 

olivine. 

Melanite  is 

almost  always 

crystallized,  and 

hence  can  be 

easily  distinguished 
from  chromite. 

./ 

Chromite  and 

picotite  can  be 

Rarely  with 
olivine 

distinguished  only 
by  chemical  means. 

and  augite. 

Common 
with  quartz, 
erthoclase, 

The  same,  but 
rare.     More 
common  in 
granulites 
and  in 
metamorphic 
(contact)  rocks. 

The  spinels  are 
distinguished  from 
magnetite  by 
their  transparency 
(in  very  thin 
sections)  and 
insolubility  in  acids. 
Pleonaste  and 
hercynite  can  be 
distinguished  only 

and  mica. 

by  chemical 

J 

analysis. 

Often  showing 
double- 
refraction  and 

With 
plagioclase, 
augite,  or 

Poor  in 
inclosures. 
Fluid 

Either  primary  (?) 
or  decomposition 
product  of 

Can  be  determined 
accurately  only 
by  chemical  tests. 

remarkable 

hornblende. 

inclosures. 

nepheline  (?) 

zonal  structure; 

Apatite 

Rare  in  the 

generally 
opaque: 
interpenetrated 
with 

needles. 

younger  basic 
eruptive  rocks, 
the  teschenites. 
As  decomposition- 

plagioclase. 

products  in 
cavities  (secondary) 

in  phonoliths, 

trachytes, 

andesites,  basalts. 

120 


DETERMINATION  OF  ROCK-FORMING  MINERALS. 


NAME. 

Chemical 

composition 
and  reactions. 

Specific 
gravity. 

Cleavage. 

Ordinary 
combinations 
and  form 
of  the 
cross-section. 

Twins. 

Color  and  power 
of  refracting 
light. 

5  Fluorite. 

CaFl2. 
Decomposed 
by 
concentrated 
H2SO4  with 
evolution 
of  HF1. 

3.1-3.2. 

Perfect 
after  O. 

In  rocks 
only  in  form 
of  minute 
angular 
granules. 

Blue, 

transparent. 
»  =  1.435- 

6.  Perowskite. 

CaTi03, 
not  attacked 
by  HC1  ; 
decomposed 
by  concen. 
H2S04. 

4.0-4.1. 

ooOc». 

In  irregular, 
arborescent, 
and 
jagged  forms, 
often  in 
sharp 
octahadra. 
(See  Fig.  56.) 

Rare; 
penetra- 
tion- 
twins. 

Grayish-violet; 
grayish- 
reddish 
broiun. 
Relief 
•well  marked. 

MINERALS   APPARENTLY  CRYSTALLIZING 


Leucite. 

Apparently 
2O2. 

Compare 
with  minerals 

of  the 

tetragonal 

system. 

1 

TABLES  FOR  DETERMINING  MINERALS. 


121 


Structure. 

Association. 

Inclosures. 

Decomposition. 

Occurrence. 

Remarks. 

Developed  in 
feldspar  and 
irregularly 
distributed 
through  the 
ground-mass. 

With  quartz, 
orthoclase, 
biotite. 

Fluid 
inclosures. 

Very  rare  ; 
secondary  in 
quartzose 
porphyries. 

The  rough 
surface 
of  the  slide  very 
characteristic. 
Grouped 
as  inclosure 
in  melilite  and 
also  in  olivine, 
often  showing 
double  refraction. 
Polarization- 
colors  very 
faint. 

With 
nepheline^ 
melilite, 
augite, 
and  olivine. 

Very  poor. 

• 

In  nepheline, 
leucite,  and 
melilite 
basalts. 

Can  be 
distinguished 
from  spinel 
by  its 
color  and 
optical 
anomalies  ; 
and  from 
garnet  by  the 
crystalline 
form. 

IN  THE  REGULAR  SYSTEM. 


Twinning 
striations,  double- 
refracting. 


122 


DETERMINATION  OF  ROCK-FORMING  MINERALS. 


II.  DOUBLE-REFRACTING 
a.  Optically-  Uniaxial 

i.  MINERALS  CRYSTALLIZING  IN 

A.  DOUBLE-REFRACTION 


NAME. 

Chemical 
composition 
and 
reactions. 

Specific 
gravity. 

Cleavage. 

Ordinary 
combina- 
tions and 
form  of  the 
cross-section. 

Twins. 

Character 
and  strength 
of  double- 
refraction. 

Polarization- 
color. 

1.  Leucite. 

K2A12Si4012. 
Soluble  in 
HCl. 
Separation  of 

2.45-2.5. 

Imperfectly 
prismatic, 
oo  P  oo  and 
oP. 

Grains,  but 
(mostly) 
crystals 
P.4P2. 

After  2P  oo  ; 
polysyn- 
thetic 
twinning- 

The  small 
individuals, 
without 
twinning- 

Not  very 
brilliant 
bluish-gray. 

pulverulent 
Si02. 
K-reaction 

Apparently 
a  regular 
form,  2O2. 

striations 
after  these 
faces, 

striations, 
apparently 
isotrope. 

with  hydro- 

Crystal 

crossing  at 

No  clear 

fluosilicic 

cross- 

right  or 

axial 

acid. 

section 

oblique 

picture 

generally 

angles. 

evident  in 

octagonal, 

(See  Fig.57.) 

c.  p.  1. 

often  nearly 
circular. 

Double- 
refraction 

more  rarely 
rectangular 

positive; 
very  weak. 

or 

hexagonal. 

2.  Rutile. 

(Nigrine, 
Sagenite.) 

Ti02. 

Ti-reaction 
with  micro- 

4-2-4-3 
(4-277)- 

ooPand 

ooPoo. 

ooP.coPoo. 

P. 
Grains; 

Very 

common 
and  charac- 

The crystals 
are  gene- 
rally too 

None 
especially 
bright. 

cosmic  bead. 

often, 

teristic 

minute  to 

however,  in 

after  Poo. 

examine 

minute, 

Bent  at  an 

with  the 

very  long 

angle  of 

condenser. 

and  narrow 

114°  25'. 

Double- 

needles  and 

Also,  a  web 

refraction 

crystals. 
The  prisms 

of  needles 
which  cut 

strongly 
positive. 

show  a 

each  other 

striation 

at  an  angle 

parallel  to 

of  about 

the  c-axis. 

60°  ._ 

Sagenite. 

Heart- 

s/taped 

twins 

according 

to  3Poo 

are  very 

* 

common. 

(Comp. 

Figs.  20 

and  59.) 

TABLES  FOR  DETERMINING  MINERA. 


MINERALS. 

Minerals. 

THE  TETRAGONAL  SYSTEM. 

POSITIVE. 


Color  and 
power  of 
refracting 

Pleo- 
chroism. 

Structure. 

Association. 

Inclosures. 

Decomposi- 
tion. 

Occurrence.' 

Remarks. 

light. 

Colorless, 
clear  as 

Aggregates 
of  spherical 

With 
augite, 

Inclosures 
of  minute 

Into  an 
aggregate 

Primary 

essential 

Easily  distin- 
guished from 

water, 
w  =  1.508. 
e  =  1.509. 

crystals 
into  a  large 
crystal. 
Zonal  and 

olivine, 
nepheline, 
plagioclase, 
and 

vitreous 
particles, 
gas-  pores, 
needles  of 

of   colorless 
or  yellow- 
ish, fine 
radial 

constituent. 
With 
sanidine, 
etc.,  in  the 

other  minerals  by 
crystalline  form, 
twinning-stria- 
tions,  and  inclo- 

radial 

sanidine. 

augite,  etc., 

filaments  or 

leucitophyrs, 

sures,  i.  e.,  their 

disposition 
of  the 

arranged  in 
zones  and 

grains  of 
zeolites. 

leucite- 
tephrites, 

regular  arrange- 
ment.     If  the 

inclosures. 

rays,  or 

Rarely 

and  basalts; 

leucite  occurs  in 

(See  Fig.  58.) 

gathered 

pseud  o- 

also  with 

very  minute 

Large 
corroded 

together  at 
the  cestre, 

morphs  of 
analcime 

nepheline 
and 

grains  through 
the  ground-mass, 

crystals 

are  charac- 

after 

plagioclase. 

it  is  often  difficult 

I.  O.  and 

teristic. 

leucite. 

Especially 

to  distinguish 

minute 

Also  rich  in 

only  in  the 

from  the  colorless 

often  sharp- 
ly formed 

inclosures 
of  other 

younger 
basic 

vitreous  base 
lying  between; 

II.  0., 

minerals,  as 

eruptive 

in  such  cases 

the  latter 

hauyn, 

rocks. 

recognized  only 

also 

augite, 

by  micro-chemical 

developed 

apatite, 

reactions. 

in  augite. 

melanite. 

Honey- 

Not  es- 

Rutile, as 

With 

Very  poor. 

As  primary 

Easily  distin- 

yellow to 

pecially 

sagenite, 

quartz, 

accessory 

guished  from 

reddish 

notice- 

often occurs 

potassium 

constituent 

zircon  by  polar- 

brown. 

able. 

regularly 

feldspar, 

very 

ization-colors, 

In  grains 
often 

developed 
in  biotite; 

garnet, 
hornblende, 

common  in 
nearly  all 

color,  and 
common   twinned 

opaque  or 

also  inter- 

omphacite. 

crystalline 

formation. 

only  trans- 

penetrated 

schists, 

lucent 

with 

especially 

(nigrine), 

ilmenite. 

those 

then  with 

Very 

containing 

. 

metallic 

common  as 

hornblende 

lustre; 

inclosure 

and  augite, 

occurring  in 

in  the 

as  the  horn- 

this  form 

minerals 

blenditesand 

but  rarely. 

accompany- 

eclogites. 

ing  it. 

As  decom- 

especially 

position- 

in  garnet 

product  of 

and 

ilmenite 

omphacite. 

secondary. 
Very 

common  in 

aluminious 

schists,  as 

"aluminous- 

schist 

needles." 

I24 


DETERMINATION  OF  ROCK-FORMING  MINERALS. 


NAME. 

Chemical 
composition 
and 
reactions. 

Specific 
gravity. 

Cleavage. 

Ordinary 
combina- 
tions and 
form  of  the 
cross-section. 

Twins. 

Character 
and  strength 
of  double 
refraction. 

Polarization- 
colors. 

3.  Zircon. 

ZrO2  +  SiO2. 
Acids  have 

4-4-4-7- 

Imperfect, 
P  and  oo  P. 

P.coPoo, 
atsoSPS; 

Rarely 
after  .Poo. 

Double- 
refraction; 

Exceedingly 
brilliant, 

no  action, 

rich  in  com- 

very 

emerald- 

except 
H2S04, 
which  decom- 

binations; 
nearly 
always  in 

rtrongly 
positive. 

green, 
hyacinth- 
red,  and 

poses  it. 

minute 

iridescent. 

but  sharply- 

defined 

crystals. 

(See  Fig. 

• 

60.) 

B.  DOUBLE-REFRACTION 


4.  Anatase. 

Like  rutile. 

3-83-3-93- 

oP  and  P. 

Sharp  />.. 

Like  rutile. 

Like  rutile. 

5.  Meionite 
Group, 
a.  Meionite. 

/3.Scapolite. 

Ca6(Ala)Si9036 

R3Al.2Si6O21 
R  =  predomi- 
nating Ca,  some 
Mg,  Na2  soluble 

2.734-2.737 
2.63-2.79. 

Perfect 

co/>oo. 

(See  Fig. 
fa.) 

Crystals 
after 

ooP.  oo/'oo  . 

P 

Double- 
refraction; 
rather 
strongly 

Brilliant 
like  quartz. 

in  HC1,  with 

and  larger 

negative. 

separation  of 
pulverulent 

grains  \  or 
elongated 

SiOa. 

prismatic 

individuals. 

TABLES  FOR  DETERMINING  MINERALS. 


125 


Color  and 
power  of 
refracting 
light. 

Pleo- 
chroism. 

Structure. 

Association. 

Inclosures. 

Decomposi- 
tion. 

Occurrence. 

Remarks. 

Colorless, 

Not 

Like  rutile, 

With 

Fluid 

Primary 

Well 

ivine- 
yelloiv; 
very  strong 
refraction  ; 

notice- 
able. 

one  of  the 
first-formed 
rock  con- 
stituents; 

quartz, 
orthoclase, 
plagioclase, 
biotite, 

inclosures, 
acicular 
cavities, 
and 

accessory 
constituent 
in  garnet, 
syenite, 

characterized  by 
crystalline  form, 
polarization- 
colors,  and 

therefore 

therefore 

hornblende, 

elongated 

quartz, 

powerful 

the  con- 

common as 

augite. 

undeter- 

porphyry, 

refraction  of 

tours  have 

inclosure 

minable 

trachytes, 

light. 

black 

in  the 

needles. 

and  many 

borders  (by 

others. 

/ 

other 

the  total 

eruptive 

reflection 

rocks,  but 

of  the 

rare; 

incident 

more 

light). 

commonly 

.0=1.92 

accompany- 

e =  1.97. 

ing  rutile 

in 

chrystalline 

•        ' 

schists. 

NEGATIVE. 


Dark 

Like 

With 

Very  rare 

lavender- 

rutile. 

quartz. 

in  granite, 

blue. 

orthoclase, 
and 

quartzose 
porphyries, 

biotite. 

and 

crystalline 

schists. 

M  =  1.594  - 
1-597 
<  =  1.558- 
1.561 

With 
sanidine, 
sodalite, 
augite. 

Primary 
accessory 
constituent; 
very  rare  in 
trachytic 

Scapolite  can 
be  easily 
distinguished 
from  orthoclase 
and  calcite  by 

Colorless. 

rocks. 

the  optical 
properties  and 

White. 

cleavage; 

w  =  1.566 
e  =  1.545. 

[Scapolite 
appears 
often  to 

With 
quartz, 
plagioclase, 

Poor; 
fluid  in- 
closures; 

Opaque, 
fibrous  ;  de- 
composed 

Rare  in 
crystalline 
sch  ists, 

meionite  is 
recognizable  by 
the  crystalline 

replace 
plagioclase 
and  to  be 

calcite, 
augite, 
garnet, 

rutile  in 
scapolite. 

into 
calcite. 

with 
plagioclase 
secondary 

form. 

a  decom- 

rutile, and 

accessory 

position- 

titanite. 

constituent. 

product 

from  it. 

126 


DETERMINATION  OF  ROCK-FORMING  MINERALS. 


NAME. 

Chemical 
composition 
and  reactions. 

Specific 
gravity. 

Cleavage. 

Ordinary 

combinations 
and  form  of 
the  cross- 
section. 

Twins. 

Character 
and  strength 
of  double- 
refraction. 

Polarization- 
colors. 

y.  Couse- 
ranite  and 
Dipyr. 

Similar  to 
scapolite; 
rich  in  the 
alkalies, 

2.69-2.76 
(2.613). 
2.62-2.68. 

According 

to  oo  /'oo. 

According 

Long 
prisms 

00/>.W/>00; 

with 

As  in 
scapolite. 
Rather 
energetic. 

Rather 
brilliant. 

H2O. 

termina- 

Not attacked 

tions 

by  acids 
(HC1),  oral 

either 
rounded  or 

least  only 

fibrous. 

with 

difficulty. 

S.  Melilite. 
(Humbold- 
ite.) 

(Ca,Mg,Na2)12, 
(Al2Fe2)2, 
Si9036. 
Easily  soluble  in 
HClwith 

2.90-2.95. 

Parallel  to 
oP  and  oo  P. 

Nearly 
always  in 
crystals; 
thin  tablets 
predomi- 

Rarely 
penetra- 
tion 
twins,  with 
chief  axes 

Double- 
refraction 
feebly 
negative. 

Not  very 
brilliant  ; 
if  yellow 
and 
fibrous, 

separation  of 
gelatinous  SiO2. 

nating, 
oP  .  oo  P  . 

00  POO. 

at  right 
angles  to 
each  other. 

shows 
polariza- 

Irregular 
grains. 
Cross- 

tion- 
colors  ;  if 
colorless, 

sections 

bluish-gray 

for  the 

polariza- 

most part 

tion- 

rectan- 

colors. 

gular, 

more 

rarely 
circular. 

TABLES  FOR  DETERMINING  MINERALS. 


127 


Color  and 

power  of 
refracting 

Pleo- 
chroism. 

Structure. 

Association. 

Inclosures. 

Decomposi- 
tion. 

Occurrence. 

Remarks. 

light. 

Bluish, 
colorless 
in  thin 

Crystals 
developed 

With 
calcite, 
actinolite, 

Very  rich; 
particles 
of  carbon, 

Fibrous 
decomposi- 
tion, with 

As  contact- 
mineral  in 
metamor- 

Can be 
distinguished 
from  chiastolite 

section, 

limestone, 

and  mica. 

quartz- 

formation 

phosed 

by  the 

clear  as 

often  rich 

grains, 

of  calcite 

limestone. 

structure  ; 

ivaier, 

in 

and 

on  the 

Very  rare. 

from 

black 

inclosures. 

leaflets  of 

crevices. 

andalusite 

from 

muscovite 

by  the 

inclosures. 

distributed 

optical 

<•>  =  1.558. 

at  random. 

properties. 

«  =  1-543- 

Generally 
lemon- 
yellow, 

ho  ney  - 

Longi- 
tudinal 
sec- 
tions ; 

Rectangu- 
lar longi- 
tudinal 
sections 

With 
nepheline, 
leucite, 
augite, 

Poor. 

The 
formation 
of  fibres 
is  a 

As 
primary 
constituent 
often 

Easily 
recognizable 
by  the 
crystalline  form. 

yellow, 

rect- 

show a 

and 

result  of 

replacing 

color,  and  fibrous 

colorless  to 
yellowish 
white. 

angles 
show  a 
very 

striation 
and 
fibrous 

olivine. 

the 
decomposi- 
tion into 

nepheline 
in  the 
nepholine 

tendency. 
If  colorless, 
easily  confounded 

feeble 
dichro- 

tendency 
parallel 

zeolitic 

substances. 

and 
leucite 

with  nepheline, 
although  the 

ism. 

to  the 

(Compare 

basalts 

hexagonal 

short 

with 

and 

isotropic  sections 

" 

sides, 

"  struc- 

lavas. 

are  wanting 

i.e.,  the 

ture.") 

in  melilite. 

chief 

Can  be 

axis  c; 

distinguished 

there  are 

from  serpentine 

also 

often  only  by 

very  fine 

the  paler  color 

spindle- 

and  the 

shaped 

interlacing 

cavities 

of  olivine 

which 

particles  ; 

appear  as 
minute 

from  biotite 
leaflets  by  the 

circles 

paler  color 

within  the 

and  the  want  of 

rounded 

dichroitic 

sections 

longitudinal 

cut  at 

sections. 

right  angles 

to  the 

chief  axis  — 

the 

so-called 

"  /  flock- 

structure.''1 

(See 

Fig.  62.) 

Developed 

with 

leucite, 

i.e., 

interpene- 

trated 

with  its 

crystal. 

128 


DETERMINATION  OF  ROCK-FORMING  MINERALS. 


2.  MINERALS  CRYSTALLIZING  IN 
A.  DOUBLE-REFRACTION 


NAME. 

Chemical 
composition 
and 
reaction. 

Specific 
gravity. 

Cleavage. 

Ordinary 
combina- 
tions and 
form  of  the 
cross-section. 

Twins. 

Character 

and  strength 
of  double- 
refraction. 

Polariza- 
tion-colors. 

1  Quartz. 

SiO-j. 
Unattacked 
bv  HC1  and 
"H2S04  : 
dissolved  by 
HF1. 

2.65 
average 
(2.651). 

Imperfect 
according 
to/?. 
The 
sections 
uneven 
owing  to 
the 
conchoidal 

Grains  or 
crystals 
R  .  —  R  or 
«>R.R.-R. 
Generally 
in  large 
ndividuals; 
regular 
hexagons, 

With 
parallel 
axial- 
rystems. 
As  a  rock- 
constituent 
never  or 
rarely 
twinned. 

Double- 
refraction 
positive. 
Strong. 

Brilliant 
yet  weak 
in  very- 
thin 
sections; 
bluish-gray, 
like 
feldspar. 

fracture. 

rhombs, 

and 

hexagons 

with  two 

parallel 

longer 

sides. 

. 

Never  as 

microlites. 

TABLES  FOR  DETERMINING  MINERALS. 


I29 


THE  HEXAGONAL  SYSTEM. 

POSITIVE. 


Color  and 
power  of 
refracting 
light. 

Pleo- 
chroism. 

Structure. 

Association. 

Inclosures. 

Decomposi- 
tion. 

Occurrence. 

Remarks. 

Colorless; 
clear  as 

Often  colored 
by  Fe303 

With 
orthoclase 

Poor  in 
mineral 

None. 
Changes 

[.  As  primary  com- 
ponent : 

In  grains 
often 

water; 
vitreous 

entering 
fissures  ; 

(and 
sanidine), 

inclosures. 
Apatite 

resulting 
from  the 

(a)  In  eruptive 
rocks  as  component 

similar  to 
sanidine, 

lustre. 

cloudy  from 

more  rarely 

prisms. 

action  of 

I.  O.     As  macro- 

but can 

to  =  1.548. 
e   =  1.558. 

numberless 
inclosures,  as 

plagioclase, 
biotite, 

In  clastic 
schists  and 

melted 
magma  are 

scopic  constituent 
in  grains  and 

be  easily 
distin- 

n the  granites 
and  the  clastic 
rocks.     Grains 

lornblende, 
and  augite. 
Never  as 

granites 
very  rich 
in  fluid 

not  un- 
common 
among  the 

crystals  with  fluid 
inclosures  in 
granites,  quartz- 

guished  • 
optically 
from  it. 

and  crystals. 
Quartz  from 
the  eruptive 

primary 
component 
in  augite- 

inclosures 
and  long 
brown  or 

quartz  of 
the  eruptive 
rocks,  or 

porphyries,  quartz- 
trachytes,  and  their 
glasses;  in  dacite 

Distin- 
guished 
from 

rocks  give 

olivine 

black,  often 

from  the 

with  vitreous 

nephe- 

evidence of 

rocks  ;  as 

bent, 

rocks 

inclosures  as  an 

line  and 

corrosion. 

also  in 

needles. 

contained 

essential  con- 

apatite by 

>eing  rounded 

nepheline 

In  the 

in  it  as 

stituent,  and  ac- 

insolu- 

and shattered; 
the  ground- 

or  leucite 
rocks. 

porphyries, 
trachytes, 

inclosures. 
Compare 

cessory  in  many 
other  eruptive 

bility  in 
HC1; 

mass  is  forced 

dacites, 

corrosion- 

rocks  ;  as  compo- 

from 

between  as 

and  other 

phenomena 

nent  of  the  11.  O.  in 

corun- 

leaves of  a 

eruptive 

and 

the  ground-mass  of 

dum  by 

book  (see 

rocks  rich 

secondary 

those  rocks  in 

the 

Fig.  43). 

in  primary 

glassy 

minute  irregular 

character 

In  granites 

glassy 

inclosures. 

grains,  never  as 

of 

commonly 

inclosures 

(See 

crystals. 

double- 

developed 

and 

Fig.  43.) 

(V)  In  nearly  all 

refrac- 

with  ortho- 

gas-pores. 

crystalline  schists  in 

tionj  from 

clase,  as 

irregular  grains 

calcite  by 

graphic- 

with  fluid  inclo- 

cleavage 

granite 

sures,  as  with  the 

and 

=  inicro- 

feldspars;  especially 

twinning. 

egniatite. 

in  gneiss,  mica- 

imp.  Fig. 

schist;  in  minute 

63.) 

grains  in  clay 

In  the 

schists. 

porphyritic 

II.  As  secondary 

eruptive 

product  through 

rocks  also 

decomposition  of 

with  radial 

silicates,  especially 

structure  as 

of  augite,  horn- 

spherulites. 

blende,  and  biotite; 

in  diabases,  as 

granular  aggre- 

gates; on  fissures 

in  lines  and 

filaments  in  many 

rocks. 

III.  In  clastic  rocks 

as  flattened  grains; 

fluid  inclosures, 

joined  together, 
reaching  to  the  very 

edges. 

With 

Inter- 

IV. As  simple  rock, 

muscovite 

twined 

building  quartzite 

and 

fluid 

and  quartz 

biotite. 

inclosures. 

schists. 

130 


DETERMINATION  OF  ROCK-FORMING   MINERALS. 


NAME. 

Chemical 

composition 
and 
reactions. 

Specific 
gravity. 

Cleavage. 

Ordinary 
combina- 
tions and 
form  of 
cross-section. 

Twins. 

Character 
and  strength 
of  double- 
refraction. 

Polarization- 
colors. 

2.  Tridy- 
mite. 

Like 
quartz. 

2.282-2.326. 

Imperfectly 
\\oP. 

In  very 
minute 
thin  tablets, 

Very 
common. 
Twinning- 

Positive. 
Very 
feeble. 

Not  very 
brilliant. 
Gray. 

pred.  oP 

plane 

and  oo  P. 

a  face  of 



\P  and  IP. 

According 

to 

v.  Lasaulx 

and 

Schuster, 

tridymite 

crystallizes 

in  the 

trielinic 

system. 

(According 

to 

v.  Lasaulx 

and 

Schuster, 

twins 

according 

to  a  plane 

of  oo  P.) 

A.P. 

differing 

but  little 

from  the 

normals  to 

oP. 

i.M.  =  c 

nearly 

Axial  angle 

65-70°. 

A  more  exact  crystallographical  and 
optical  determination  of  tridymite 
is  generally  impossible,  owing  to  the 
minuteness  of  the  crysta  s  occurring 
in  the  rocks.     Microscopical    tridy- 
mite behaves  like  an  hexagonal  rain- 

eral  in  p.p.l. 

TABLES  FOR  DETERMINING  MINERALS. 


Color  and 
power  of 
refracting 
light. 

Pleo- 
chroism. 

Structure. 

Association. 

Inclosures. 

Decomposi- 
tion 

Occurrence. 

Remarks. 

Colorless, 
clear  as 
water. 
«,  i.e. 
/3  =  1.4285. 

Generally 
in 
Aggregates 
of  minute 
thin  tablets; 
either 

With 
quartz, 
sanidine, 
plagioclase, 
augite, 
biotite, 

Fluid 
inclosures. 

Primary  as 
accessory 
constituent, 
and 
secondary 
as  decom- 

The tendency  to 
form  aggregates 
is  very 
characteristic  for 
microscopic 
tridymite;  the 

hexagonal 
or 

and 
horn- 

position- 
product  in 

optical  properties 
and  the 

irregular 
tablets, 
lapping 

blende. 
Secondary 

rhyolites, 
trachytes, 
hornblende- 

twinnings  for 
the  larger 
crystals  and 

over 

with  opal 

and  augite- 

those  suitable 

each  ft  her 

and  chalce 

andesites. 

for  optical 

like 
sh  ingles. 

dony. 

Exceed- 
ingly 

investigations. 

Often  in 

common 

the 

in  the 

neighbor- 

younger 

hood  of 

acidic 

feldspars, 

rocks  ; 

or  in  large 

rare  in  the 

groups 

basic  older 

in  the 

rocks. 

ground- 

Secondary 

mass. 

in  cavities 

(See 

of  these 

Fig.  64.) 

rocks,  and 

then 

generally 
in  larger 

tablets. 

132 


DETERMINATION   OF  ROCK-FORMING  MINERALS. 


B.  DOUBLE-REFRACTION 


NAME. 

Chemical 
composition 
and 
reactions. 

Specific 
gravity. 

Cleavage. 

Ordinary 
combina- 
tions and 
form  of 
cross-section. 

Twins. 

Character 
and 
strength  of 
double- 
refraction. 

Polarization- 
colors. 

1,  Calcite. 

CaC03. 

Easily 
soluble 
in  HCI, 
with  rapid 
evolution 
of  CO2. 
Easily 

2.6-2.8 
(2.72).    ( 

Perfect 
R. 
(See 
Fig.  65.) 

Only  in 
regular 
grains 
and 
crystalline 
aggregates. 

Poly- 
synthetic 
twinning- 
striation 
after 
-\R. 
(See  Figs. 
21  and  65.) 

Double- 
refraction 
very 
strongly 
negative. 

Generally 
weak;  gray; 
yet  often 
brilliantly 
iridescent 
like  talc, 
especially 
in  those  cases 

soluble  in 

where 

acetic  acid. 

the  calcite 

occurs  in 

minute 

granules. 

I 

\ 

2.  Dolomite. 

(Ca)  (Mg) 
CO3. 
More 
difficultly 
soluble  in 
acids  than 
calcite. 

2.85-2.95. 

ditto. 

Grain  sand 
R. 

See  "  Re- 
marks." 

ditto. 

3.  Mag- 

ditto. 

nesite. 

MgC03. 

2.9-3.1. 

ditto. 

ditto. 

More 

difficultly 
soluble  in 

HCI. 

4.  Siderite. 

FeC03. 
Soluble  in 

3-7-3-9- 

ditto. 

ditto. 

ditto. 

HCI  with 

evolution  of 

gas. 

TABLES  FOR  DETERMINING  MINERALS. 


133 


NEGATIVE. 


Color  and 

power  of 

refracting 

Pleo- 

chroism. 

Structure. 

Association. 

Inclosures. 

Decomposi- 
tion. 

Occurrence. 

Remarks. 

light. 

Colorless, 
white, 

Feeble 
absorption. 

Generally 
in   granular 

In  nearly 
all  rocks 

Fluid 
inclosures  ; 

None. 

As  primary 
constituent, 

Well 
character- 

grayish. 

aggregates, 

bearing 

very  poor 

building  by 

ized  by 

w  =  1.6543 
e  =  1.4833 

in  cavities; 
in  threads 
and  fibres. 

augite, 
hornblende, 
biotite, 

in 
mineral 
inclosures. 

itself  a 
simple  rock, 
limestone; 

the  rhombo- 
hedral 
cleavage 

In  irregular 

and 

not  yet 

and 

grains, 

plagioclase. 

assuredly 

twinning- 

single 
individuals. 

proved  as 
such  in 

striation. 
If  occurring 

between 

eruptive 

in  minute 

the  other 

rocks. 

granules, 

con- 

Here as 

often 

stituents. 

deco  m  posit  io  n 

difficult  to 

Product, 

accurately 

especially  of 

determine; 

the  bisilicates 

the 

and  of  mica, 

solubility 

where  it 

and 

occurs  in 

polarization 

lenticular 

colors 

forms 

remain  as 

between  the 

means  of 

laminae. 

recognition. 

Primary  and 
secondary  in 

crystalline 

schists. 

ditto. 

ditto. 

The  poly- 

synthetic 

twinning- 

striations 

after  —  \R 

are  wanting 

on  dolomite. 

With  olivine 

ditto. 

ditto. 

With 

and 

serpentine. 

serpentine  as 

decomposi- 
tion-product. 

Magnesite 

and 

siderite 

can  be  dis- 

Yellowish 

ditto. 

See 

As  decomposi- 

tinguished 

to  brown. 

Calcite. 

tion-product 

from 

in  small 

calcite 

balls  of  con- 

only by 

centrically- 

chemical 

arranged 

means. 

layers  and 

with  radial 

fibres;  in 

andesites,  etc 

134 


DETERMINATION  OF  ROCK-FORMING  MINERALS. 


NAME. 

Chemical 
composition 
and 
reactions. 

Specific 
gravity. 

Cleavage. 

Ordinary 
combina- 
tions and 
form  of  the 
cross-section. 

Twins. 

Character 
and  streujfth 
of  double- 
refraction. 

Polarization- 
color. 

5-Nepheline 
Group, 
a.  Ekeolite. 

2.65  (2.591). 

Imperfect; 
coarse 
fissures. 

Only  in 
larger 
grains. 

Generally 
bluish  gray, 
not  very 

brilliant. 

(Na,  K)2 

Al2Si3O8. 
Easily  soluble 
inHCl 
with 
separation  of 
gelatinous 
SiO2;  on 
evaporation 
cubes  of 
NaCl. 

Double- 
refraction 
negative, 
not 
energetic. 

Similar  to 
the  feldspar 
in  very 
thin  sections. 

/3.  Nephe- 
line. 

(2.58-2.65) 
2.56. 

Imperfect 
fffand 

00  P. 

Hexagons 
and 
rectangles; 
in  -minute 
crystals 

aoP.oP, 

in  short 

prisms, 

and  in 

minute 

irregular 

granules. 

(See 

Fig.  66.) 

TABLES  FOR  DETERMINING  MINERALS. 


135 


Color  and 

power  of 
refracting 

Pleo- 
chroism. 

Structure. 

Association. 

Inclosures. 

Decomposi- 
tion. 

Occurrence. 

Remarks. 

light. 

Gray, 
reddish 
brown, 
fatty  lustre; 
colorless 

Irregular 
grains 
inter- 
penetrated 
with 

With 
sodalite, 
microline, 
hornblende, 
titanite. 

Poor; 
often 
colored 
green  by 
hornblende. 

Fibrous, 
zeolitic 
meta- 
morphosis. 

As  primary 
essential 
constituent 
in  the  older 
eruptive 

Well  recognizable 
macroscopically. 
The  solubility 
and  Na-reaction 
are  characteristic. 

in  thin 

other  con- 

rocks; in  the 

section. 

stituents. 

elaeolite- 

syenites. 

Colorless, 
clear  as 

In  crystals 
or  in 

With 
leucite, 

Inclosures 
of  augite 

Generally 
fresh,  in 

As  primary 
essential 

Distinguished 
from:  apatite  by 

water, 
w  =  1.539 

aggregates 
of  minute 

augite, 
olivine, 

very 
commonly 

phonolites 
more  often 

constituent 
in  the 

the  imperfect 
cleavage, 

-  1.542 
e  =  1-534 

irregular 
granules. 

or  with 
sanidine 
and  augite, 
or  with 
hornblende 

arranged 
parallel 
to  the  faces. 
(See 
Fig.  66.) 

opaque 
and  meta- 
morphosed 
into 
zeolites  ; 

younger 
eruptive 
rocks: 
nephelinites, 
nepheline- 

microchemical 
reactions,  and 
crystalline  forms, 
as  apatite 
commonly 

and 

then 

and  leucite- 

shows  /", 

titanite. 

polarizing 

basalts, 

besides  °°P,  oP, 

like  an 

phonolites 

and  occurs 

aggregate. 

and 

in  long  prisms; 

tephrites. 

quartz  never 

occurs  in  such 

minute  crystals 

as  nepheline; 

feldspar-th  reads 

are  long 

and  twinned; 

melilite  has  no 

hexagonal 

transverse 

sections; 

zeolites  generally 

evidence  their 

secondary 

character. 

The  short 

rectangular 

longitudinal 

sections  are 

wanting  in 

tfidymite. 

If  nepheline 

occurs  in 

aggregates  of 

minute  granules, 

then  it  is 

similar  to  a 

colorless  vitreous 

mass  or  quartz 

and  orthoclase 

aggregates, 

and  can  be 

proven  only  by 

microchemical 

tests. 

136 


DETERMINATION  OF  ROCK-FORMING  MINERALS. 


NAME. 

Chemical 
composition 
and  reactions. 

Specific 
gravity. 

Cleavage. 

Ordinary 
combinations 
and  form  of 
the  cross- 
section. 

Twins. 

Character 

and  strength 
of  double- 
refraction. 

Polarization- 
colors. 

y.  Cancri- 

nite. 

Composition 
like  a 

2.448-2.454. 

Imperfect 

Larger 
irregular 

Double- 
refraction 

Rather 
brilliant; 

nepheline, 

grains. 

negative. 

aggregate 

poor  in 

polariza- 

potassium, 

tion. 

together  with 

CaO,  C02, 

and  H2O. 

Soluble  in 

HC1  with 

effervescence, 

with 

separation  of 

gelatinous 

Si02. 

5.  Lieben- 

erite. 

Potassium- 
aluminium 

2.799-2.814. 

Very 
imperfect 

Only  larger 
crystals 

Double- 
refraction 

Remark- 
able 

silicate. 

CO/*. 

oo  P  .  oP. 

negative  (?) 

aggregate 

H2O,  traces 
of  Fe,  Mg. 

Cannot  be 
confirmed, 

polariza- 
tion^ -very 

Similar  to 

as  the 

brilliant. 

pinite, 

crystals 

incompletely 
decomposed 

are  always 
completely 

by  HC1. 

decom- 

posed. 

6.  Apatite. 

CaBP3012Cl. 
Ca5P3012Fl. 
Soluble  in 
acids. 
Reaction  for 
phosphoric 
acid. 

3.16-3.22. 

Crystals, 
more  rarely 
grains. 
Imperfect 
cleavage 
parallel  oP 
and  oo  P. 

™P.P 
and  more 
rarely  oP. 
Generally 
long  prisms. 
(See 
Fig.  67.) 

Double- 
refraction 
negative, 
feeble. 

Generally 
not  very 
brilliant, 
as  with 

nepheline. 

Remarkable 

"separa- 

tion" 

(abson- 

derung) 

II  oP,  the 

acicular 

crystals 

thereby 

separating 

into  several 

members. 

CABLES  FOR  DETERMINING  MINERALS. 


137 


Color  and 

power  of 
refracting 

Pleo- 
chroism. 

Structure. 

Association. 

Inclosures. 

Decomposition. 

Occurrence. 

Remarks. 

light. 

Lemon- 

Fibrous, 

Like 

Poor. 

Fibrous 

Like 

Can  be 

yelloiv, 
nearly 

inter- 
penetrated 

elaeolite. 

Leaflets  of 
ferric  oxide, 

decomposi- 
tion with 

elaeolite; 
rare. 

dis- 
tinguished 

colorless, 

with 

etc.,  like 

formation  of 

from 

in  sections. 

nepheline 

elaeolite. 

calcite. 

elaeolite 

and 

(Cancrinite 

only 

orthoclase. 

appears  to  be 

macro- 

only  a 

scopically 

decomposed 

and  by  the 

nepheline} 

content  of 

CaCOg. 

Oil-green; 
in  sections 
colorless; 

Only  as 
"spring- 
ling"b 

With 
flesh-red 
orthoclase 

It  is  probably 
a  completely 
decomposed 

Rare. 
In 
orthoclase- 

Easily 
recog- 
nizable 

white. 

in  macro- 

and mica. 

nepheline(?)  or 

porphyry. 

by  the 

scopic 
crystals. 

cordierite  (?)• 
consists 

crystalline 
form  and 

principally  of 

decom- 

minute 

position. 

m  uscovite 

leaflets,  which 

i.p.l.  appear 

very  clearly. 

Colorless, 
white; 
colored 
reddish, 

If 
colored, 
plainly 
di- 

In  irregular 
grains,  or 
inelongated 
often  very 

Proven 

in  nearly 
all  rocks. 

Vitreous 
inclosures, 
gas-pores.  Very 
characteristic 

Always  fresh. 

As 
accessory 
constituent 
in  nearly 

Dis- 
tinguished 
from: 

nepheline 

brown, 

chroitic. 

narrow 

are  inclosures 

all  eruptive 

especially 

black, 

columns, 

of  black  or 

rocks  and 

by  the 

from 

broken  as  a 

brown  needles, 

crystalline 

micro- 

numberless 

con- 

or minute  dust- 

schists. 

chemical 

inclosures; 

sequence  of 

like  granules. 

One  of  the 

reactions 

not  water- 

^ 

the  basic 

which  permeate 

minerals 

(comp. 

clear  like 
nepheline; 

separation. 
Inclosures  ! 

the  whole 
crystal;  in  this 

first 
eliminated 

nepheline), 
inclosures 

more 

(See 

respect,  in  the 

•    in 

very  char- 

prominent 

transverse 

formation 

acteristic; 

among  the 

Only 

sections 

of  the 

hattyn  in 

rock-con- 

accessory 

especially, 

rocks. 

the  longi- 

stituents. 

constituent. 

showing  a  great 

tudinal 

n  =  1.657. 

As 

similarity  to 

sections 

in  closure 

many  hauyns 

and  basic 

especially 

also  rich  in 

separations; 

common 

inclosures. 

olivine  by 

in  the 

Central 

the  optical 

bisilicates 

inclosures  of 

properties 

hornblende 

glass,  etc.,  often 

and  the 

and 

with  the  form 

separation. 

biotite. 

of  apatite. 

138 


DETERMINATION  OF  ROCK-FORMING  MINERALS. 


NAME. 

Chemical 
composition 
and 
reactions. 

Specific 
gravity. 

Cleavage. 

Ordinary 
combina- 
tions and 
form  of  the 
cross-section. 

Twins. 

Character 

and  strength 
of  doublu- 
refractioii. 

Polarization- 
colors. 

7.  Corun- 
dum. 

A1203. 
Insolu'ile 

3-9-4- 

R  and  oR. 

oo  />2  .  0R. 
R  and  grains. 

In  rocks 
seldom  (?) 

Double- 
refraction 

Very 
brilliant. 

in  acids. 

Hexagons 

strongly 

and 

negative. 

rectangles 

whose  angles 

are 

truncated 

by/?. 

8.  Tourma- 
line. 

(Schorl.) 

Very 
complicated. 

R6A12B2 

SuO* 

+R3(A12)2.B2 
Si4O20 

R  =  Na 

predominat- 
ing and 

R  =  Mg,  Fe. 

3-°59- 

Imperfect 
R  and  <x>/?2. 
Separation 
\\oR. 
Very 
perfect. 

Almost  only 
in  crystals. 
<x>R 
/?.oo/>2.  -. 

•2 

Transverse 
sections 
three-,  JZJT-, 
and  nine-sided 
when  observed 
parallel  to 
the  chief  axis; 
often  showing 
good  hemi- 
morphism. 

Double- 
refraction 
negative, 
energetic. 

Rather 
brilliant; 
between 
brown  and 
red. 

Contains 
boric  acid. 
Not  attacked 
by  acids. 

oR  below, 
R  above. 
(Comp.  Figs. 
68  and  69.) 

9.  Haema- 
tite. 
(Eisen- 
glanz.) 

Fe2O3. 

Easily  soluble 
in  HC1. 

5.19-5.28. 

R.oR. 
Not  charac- 
teristic in 
microscopic 
individuals. 

Mostly  tabular 
thin  leaflets, 

<7/?.00/>2, 

and  irregular 
leaflets. 

With 
parallel 
axial 
systems. 
Penetration 

Indetermin- 
able, as 
occurring 
always  in 
exceedingly 

Not  very 
brilliant. 

twins 

thin 

with  re- 

leaflets in 

entrant 

crevices 

angles. 

in  minerals. 

TABLES  FOR  DETERMINING  MINERfyS^                     139 

. 

Color  and 
power  of 
refracting 

Pleo- 
chroism. 

Structure. 

Association. 

Inclosures. 

Decom- 
position. 

light. 

Colorless, 
azure-blue. 

If 

colored, 

Rough  section- 
sitvface. 

With  quartz, 
orthoclase, 

Very  poor; 
fluid  and 

Very  rare. 
Contact- 

When 
in  grains 

spotted. 

<ery  strong. 

In  rounded 

and  biotite; 

vitreous 

mineral,  in 

similar  to 

also  brown 

to  =  azure- 

grains  or 

pleonaste, 

inclosures; 

metamorphic 

apatite,  yet 

from 
inclosures; 

blue, 
e  =  sea- 

short  prisms. 
One  of  the 

andalusite. 

zircon. 
Brown 

schists  and 
in  trachytes. 

recognizable 
by  the 

powerfully 

green. 

ninerals   first 

dust-like 

brilliant 

refracting 
light. 

separated. 
Zonal 

inclosures 
(so-called 

polarization- 
colors. 

appearing 
well  in 

structure, 
blue  core, 

ordinary 
corundum). 

listinguished 
rom:    quartz 

sections. 

and  colorless 

by  the  rough 

to=  1.768 

layers. 

surface  and 

e  =  1.760. 

character  of 

the  double- 

refraction, 

also  not  so 

clear  as 

quartz; 

olivine  by 

the  optical 

properties. 

Mostly 
grayish 
blue, 

Very 
marked 
dichroism. 

In 
macroscopic 
and  in 

With  quartz, 
orthoclase, 
and 

Very  poor. 
Fluid 
inclosures. 

Common 
as  primary 
constituent. 

Easily 
recognizable 
>y  crystalline 

brown. 

to  =  dark 

exceedingly 

muscovite 

In  certain 

form  and 

to  =  1.64 

e  =  T.02. 

>lue,  brown 
to  black, 
e  =  light 

minute 
crystals, 
rarely  in 

in  granite. 
With  quartz, 
orthoclase. 

granites  in 
grains. 
Accessory  in 

pleochroism. 
Distinguished 
from  : 

blue  to 
light  gray 
and  brown. 

granules,  as 
in  certain 
metamorphic 
rocks.     In 

mica,  and 
other 
accessory 
minerals,  as 

many  crystal- 
line schists, 
especially 
clay-schists, 

hornblende  by 
the  optical 
properties; 
biotite  by  the 

granites  in 
grains  inter- 
penetrating 
quartz.    The 

staurolite 
and  garnet. 

also  in 
clastic  rocks. 
Finally, 
common  and 

repeated 
formation  of 
laminae. 

larger 

characteristic 

individuals 

. 

in  schists 

often  in 

metamor- 

radial fibres 

phosed  by 

at  the 

contact  with 

terminations. 

eruptive 

rocks. 

especially 

granite. 

Iron-black 
with 

Occurring 
only  in  leaves 

In  nearly 
all  rocks 

Into 
brown- 

Compare 
association. 

Easily 
recognizable 

metallic 

infiltrated  on 

especially 

ish-red 

A  ccessory 

by  the  form 

lustre;  in 
thin  leaves 
blood-red, 
also 

the  fissure 
of  minerals. 
Is  always  a 
secondary 

with 
decomposed 
biotite, 
hornblende, 

and 
brown 
pulver- 
ulent 

secondary 
mineral. 

of  occurrence 
and 
blood-red 
color. 

yellowish 

mineral. 

augite, 

ferric 

red  to 

Only  in  some 

magnetite, 

hydrox- 

brownish 

basalts  also 

as  decom- 

ide. 

violet. 

appearing  as 

position- 

primary. 

product. 

140 


DETERMINATION  OF  ROCK-FORMING  MINERALS. 


C.  MINERALS  APPARENTLY 

1.  Biotite;  in  brown,  apparently  isotrope  (optically -uniaxial), 

2.  Chlorite;  in  green,  apparently  isotrope  (optically-uniaxial), 

II.  b.  Optically-Biaxial 

i.  MINERALS  CRYSTALLIZING  IN 

o-.  No  INTERFERENCE-FIGURE  VISIBLE 


NAME. 

Chemical 
composition 
and 
reactions. 

Specific 
gravity. 

Cleavage. 

Ordinary 
combinations 
and  form  of 
the  cross- 
section. 

Twins. 

Optical 
orientation. 

Character 
and  strength 
of  double- 
refraction. 

Direction 
of 
extinction. 

Tabular 

1.  Olivine. 
(Chryso- 
lite.) 

«Mg2SiO4 
+  FeSi04 
Rather 
easily 
soluble  in 

3-2-3-4- 

Perfect 

II  oo  Poo, 

imperfect 

00  ^00. 

Con- 

crystals. 
<*>P._Pv>. 
ooPoo. 

Hexagonal 
cross- 

Very 
rare 
according 

ft. 

i.  M.  -L 

OO/OO 

a  =  C. 

Z  =  a. 

Double- 
refraction 
very 
strongly 
positive. 

HC1,  with 
separation 

,  °f- 
gelatinous 

Si02. 

choidal 
fracture 
not  so 
evident 
in 

sections 

00/>=I30°2'. 

Poo  =  76°54' 
or  large 
rounded 

also  pene- 
tration- 
twins. 

Feeble 
dispersion 
of  the 
axes. 

sections. 
Gene- 

grains. 
(See 

P   <  7', 

large 

rally 
twisted 

Fig.  71.) 

angle. 

crevices. 

• 

TABLES  FOR  DETERMINING  MINERALS. 


HEXAGONAL. 

hexagonal  leaflets;  small  axial  angle.     See  Monoclinic  System. 

hexagonal  or  irregular  leaflets  ;  small  axial  angle.    See  Monoclinic  System. 

Minerals. 

THE  RHOMBIC  SYSTEM. 

(opt.  A. P.  \\oP)  IN  SECTIONS  ||  oP. 


Polariza- 
tion- 
colors. 

Color  and 
power  of 
refracting 
light. 

Pleo- 
ohrolam. 

Stmcture. 

Associa- 
tion. 

Inclosures. 

Decomposition. 

Occurrence. 

Remarks. 

Exceed- 

Colorless ; 

Possesses 

Princi- 

Rather 

Most 

As  primary 

Distin- 

ingly 

in 

a  rough 

pally  with 

poor; 

commonly 

essential 

guished 

brilliant, 

sections 

section- 

augite, 

besides 

into 

constituent 

from  : 

stronger 
than  in 
augite. 

rarely 
greenish  ; 
rough 
section- 
surfaces. 
Relief 

surtace  ; 
the  manner 
of  deco  tit- 
posit  ion 
into 
yellowish- 

plagio- 
clase, 
nephe- 
line, 
leucite. 
Also  with 

the 
minute 
brown 
picotite 
octa- 
hedra, 

serpentine 
(compare 
aggregates), 
whereby  the 
picotite 
inclosures 

in  all 
basaltic 
rocks   in 
olivine- 
felsand 
picrite, 

quartz  in 
iso  trope 
sections 
easily  ; 
zoisite  by 
the 

marked. 

red,  brown 

horn- 

vitreous 

remain. 

melaphyr, 

crystalline 

|3  =  1.678. 

or  greenish 

blende 

and 

Also  into  a 

olivine- 

form  (never 

serpentine, 
beginning 
at  the 

and 
biotite. 
Almost 

rarely 
fluid 
inclo- 

brown fibrous 
aggregate  ; 
into  picntes 

gabbro, 
olivine- 
norite, 

in  long 
needles) 
and  polari- 

crevices, is 

never 

sures, 

and  pseudo- 

olivine- 

zation- 

charac- 
teristic ; 

with 
primary 

magne- 
tite,  very 

morphs  of 
calcite  alter 

diabase. 
(In 

colors  ; 
colorless 

also  the 
inclosures 

quartz  or 
ortho- 

rarely 
other 

olivine. 
By  the  de- 

crystalline 
schists  ?) 

a  ugite 
by  the 

of  picotite 

clase. 

mineral 

composition, 

Also  in 

cleavage  in 

(see 

inclo- 

elimination 

certain 

sections 

Fig.  70). 

sures. 

of  ferric 

mica- 

at  right 

Partly  in 

hydroxide, 

porphyries. 

angles  to 

sharp 

and  magne- 

the c-axis  ; 

crystals, 

tite  in  the 

sanidine 

partly  in 

crevices. 

by  the 

fragments 
of  them, 

Totally 
decomposed 

rough 
surface  and 

or  in 

olivine, 

the  ex- 

irregular 
grains. 

very  rich  in 
iron,  always 

ceedingly 
brilliant 

Constituent 
I.  0.  in 

in  sharp 
tabular 

polariza- 
tion-colors. 

vitreous 

crystals  in 

eruptive 

limburgite 

rocks  ; 

from 

also  in 

Sasbach, 

minute 

was  called 

crystals, 

hyalosiderite. 

otherwise 

only  as 

"spring- 
ling." 

142  DETERMINATION  OF  ROCK-FORMING  MINERALS. 

fi.  AXIAL  PICTURE  VISIBLE  IN  SECTIONS  ||  oP. 
aa.   Appearance  of  the  I  (-J-) 


NAME. 

Chemical 
composition 
and 
re  ctions. 

Specific 
gravity. 

Cleavage. 

Ordinary 
combinations 
and  form  of 
the  cross- 
section. 

Twins. 

Optical 
orientation. 

Character 
and  st.  ength 
of  double- 
refraction. 

Direc- 
tion of 
extinc- 
tion. 

1.  Silli- 

manite. 

A1aSi06. 

Not 

3.23-3.24. 

||  00^00 

Separa- 

Prismatic 
individuals 

A.  P.  ||  oo  A» 

i.M.  X  oP. 

Double- 
refraction 

attacked 

tion  of 

without 

c'  =  c. 

positive. 

by  acids. 

the  thin 
needles 

defined 
termina- 

Z =  a. 

^    t. 

- 

\\oP. 

tions. 

00  P    =    III0. 

(See 

d  —  0. 

Small 
axial  angle 

Fig.  72.) 

p  =  44°. 
Strong 

dispersion. 

2.  Stau- 
rolite. 

H2R3(A12)6 
Si6O34. 
R  =  3Fe  + 
iMtr. 
Insoluble 
in  acids. 

3-34-3-77- 

Perfect 

CO/*  00, 

imperfect 

00  P. 

Separa- 
tion 
\\oP. 

Rarely 
irregular 
grains. 
Crystals 
ooP.  oofoo 
oP. 
oo/>  =  128° 

42'. 

Penetra- 
tion- 
twins^ 
wherein 
the  c-axes 
cut  each 
other 
at  right 
or  nearly 

A.  P.  ||  oo  .Poo 

i.  M.  -L  oP. 

c'  =  C. 

£  =  a. 
a  =  b. 
(Dispersion 
feeble 
P>  v.) 

Double- 
refraction 

positive, 
strong. 

right 

• 

angles; 

rarely 

micro- 

scopic. 

(See 

Figs.  22 
and  73.) 

TABLES  FOR  DETERMINING  MINERALS. 
DOUBLE-REFRACTION  IN  THEM  POSITIVE. 
Middle  Line  on  oP. 


143 


Polariza- 
tion- 
colors. 

Color  and 
power  of 
refracting 

Pleo- 

chroism. 

Structure. 

Association. 

Inclosures. 

Decom- 
position. 

Occurrence. 

Remarks. 

light. 

Very 
brilliant, 
some- 
what 

Colorless, 
often 
colored 
red  by 

In 
exceedingly 
long,  thin 
needles, 

With 
quartz, 
orthoclase, 
biotite,  and 

Very  poor. 
Fluid 
inclosures. 

As  primary 
accessory 
constituent 
in 

Distin- 
guished 
from  : 
zoisite 

like 

Fe203 

generally 

muscovite. 

crystalline 

by  the 

musco- 

on 

in  large 

schists; 

character 

vite. 

fractures. 

numbers, 

rare. 

of  the 

often 

double- 

fituly 

refraction 

fibrous 

and  polar- 

and ar- 

ization- 

ranged 

colors; 

parallel; 

andalusite 

developed 

by  the' 

in  quartz, 

character 

cordierite, 

of  the 

and  other 

double- 

minerals. 

refraction 

(See 

and 

Fig.  77.) 

cleavage. 

Very 
brilliant. 

Light  or 
deep 
yellowish 
brown. 
Relief 
very 
marked. 

Easily 
recogniz- 
able, 
especially 
in  the 
longitu- 
dinal 

In  large 
and  small 
crystals 
the  num- 
berless in- 
closures are 
character- 

With 
quartz, 
orthoclase, 
mica,  and 
garnet. 

Inclosures 
of  minute 
quartz 
grains, 
bitumen, 
hematite, 
are 

As  primary 
accessory 
constituent 
in 
crystalline 
sch  ists, 
especially 

Well 
character- 
ized by  the 
color  and 
pleochro- 
ism. 

/3p==  1-7526 

sections. 

istic. 

common. 

mica- 

C  =  dark 

schists. 

brown. 

a=b 

with 

slight 

difference 

=  light 

yellow. 

144 


DETERMINATION  OF  ROCK-FORMING  MINERALS. 


NAME. 

Chemical 

composition 
and 
reactions. 

Specific 
gravity. 

Cleavage. 

Ordinary 
combinations 
and  form 
of  the 
cross-section, 

Twins. 

Optical 
orientation. 

Character 
and  strength 
of  double- 
refraction. 

Direc- 
tion of 
extinc- 
tion. 

3.  Ensta- 

tite. 

MgSi03. 
Only  with 
difficulty 
attacked 
by  acids. 

(3-I53)9 

Perfect 

w5»! 

oo/oc. 

Longi- 
tudinal 

Long 
prismatic 

00  P  .    00  /OO 

oo/oo  .mPo=. 
Octagonal 
transverse 

N.B    That 
position  is 
selected 
where 

00/>=92° 

lies  to  the 

Double- 
refraction 
positive, 
rather 
strong. 
Weaker  (?) 

sections 
inclining 

sections 
with  two 

front  : 

A.  P.    1    00/00 

than  in 
monoclinic 

to 
fibrous. 

controlling 
Pairs  ff 

i.  M.  -L  0P 

c'  =  C. 

augite. 

oo  P  about 

92°. 

faces, 
similar  to 

b   =  b. 

Separa- 
tion 
\\oP. 

monoclinic 
augite. 

#  =  a. 
(See 
Fig  7.) 
Like 

bronzite. 

The 

positive 
axial  angle 

increasing 

with  iron 

present. 

Dispersion 

not  strong. 

P  >  v% 

and  clear. 

[Comp. 
Zirkel.  Min. 

p.  597.     Ac- 

cording to 

Tscher- 

mak's 

position  the 

optical 

orientation 

in  enstatite 

and 

bron/ite  is 

the 

following: 

A.  P.  ||  00  /CO 

c1  =  C.  i.  M. 

b  =a. 

d  =  b.] 

(See 

• 

Fig.  74-) 

TABLES  FOR  DETERMINING  MINERALS. 


145 


Polari- 
zation- 
colors. 

Color  and 

power  of 
i  ell-acting 
light. 

Pleo- 

eliroism. 

Structure. 

Association. 

Inclosures. 

Decomposi- 
tion. 

Occurrence. 

Remarks. 

Very 
bril- 
liant. 

Colorless 
to 
greenish. 
Relief 

In  irregular 
elongated 
prismatic 
individuals 

With 
plagioclase, 
olivine, 
and 

Very  poor. 

Into 
serpentine 
with 
formation 

As  essential 
and 
accessory 
constituent 

Distin- 
guished 
from: 
olivine 

marked. 

which 

monoclinic 

of  talc. 

in  basic 

by  the 

show 
lira 
longi- 
tudinal 

augite. 

Into  bast  it  e 
(compare). 
Decompo- 
sition 

porphy- 
ritic 
eruptive 
rocks. 

fibrous 
tendency 

IU-; 
zoisite 

striation 

resembles 

With 

by  the 

like 

the  meta- 

olivine in 

character 

fibres. 
(See 

morphosis 
of  olivine 

olivine-fels. 
Rare  in 

of  the 
double- 

Fig.  75.) 
More 
rarely  in 
crystals, 
generally 

into 
serpentine, 
yet  mostly 
crystalline 
outlines 

quartzose 
rocks  as 
quartz- 
porphy- 
ntes;  in 

refraction 
and 
polariza- 
tion-colors; 
si  Hi  ma  «- 

decom- 

obtained. 

porphy- 

ite  by  the 

posed. 

rites, 

form 

More  often 

diabase- 

(never  in 

interpene- 

porphy- 

so  minute 

trated  ||  c 

rites, 

needles) 

with 

melaphyrs  ; 

and 

monoclinic 
augite. 

also  in 
gabbro 

cleavage  ; 
the  follow- 

and nonte. 

ing 

minerals 

only  by  the 

variation 

of  the 

contained 

iron. 

146 


DETERMINATION  OF  ROCK-FORMING  MINERALS. 


NAME. 

Chemical 
composition 
and 
reactions. 

Specific 

gravity. 

Cleavage. 

Ordinary 

combinations 
and  form  of 
the  cross- 
section. 

Twins. 

Optical 
orientation. 

Character 
and  strength 
of  double- 
refraction. 

Direction 
of 
extinction. 

4.  Bronzite. 

;«(MgSiO3) 

n  (Fe~Si03). 
Not 

3-3-5 
(3.12- 
3-25)- 

Perfect 

and 
separates 

Elongated 
acicular. 

00/>.    00/00 
00/>00 

Not 
rarely  the 
large 
bronzite 

Like 
enstatite. 
[According 
to 

Double- 
refraction 
positive, 
like 

attacked 
by  acids. 

according 
to 

predomi- 
nating; 

cleavage- 
leaves 

Tschermak 

emtatite. 
In  sections 

oo  Pea. 

Q£f*    ^>/oo 

||   00/00 

or 

at  right 

very 

bent, 

c  —  a  1 

angles  to 

similar 

wavy, 

Large  axial 

the  middle 

to  the 

through 

angle, 

lines  (i.e., 

tnonoclinic 

repeated 

6a0-9o°. 

Il^-Pand 

augite. 

twinning 
after 

Inter- 
mediary 
product 

oo/oo)  only 
an  opening 
cross,  with 

Rarely  in 

between 

traces  of 

por- 

enstatite 

the  lemnis- 

phyritic 
eruptive 

and  hyper- 
sthene. 

cates.  is 
visible. 

rocks 

In  sections 

knee- 

at  right 

shaped 

angles  to 

twins 

an  optic 

after 

axis  one  or 

;;/  /oo  in 

no  ring  is 

asteroid 

visible. 

crystal- 

•     line 

groups. 

5.  Antho- 
phyllite. 

n  (MgSiOg) 

3-187- 
3.225. 

||  00/00 

oo/>, 

In  leaf-like 
masses, 

A.  P.  = 

00/00 

Double- 
refraction 

Not    3' 
attacked 
by  acids. 

00/00 

imper- 
feet 

very  rarely 
in  crystals. 
Transverse 

i.M.  i_oP 

&  =  b 

strongly 
positive. 

[Accord- 

sections 

a  =  a. 

ing  to 
Tscher- 
mak 
imperfect 

like  those 
of 
monoclinic 
hornblende. 

Dispersion 
clear  about 
C  =  -v  >  p; 
large  axial 

||   00/00, 

angle. 

con- 
choidal 

(See  Fig.  8.) 

separa- 

tion after 

oo/o,.] 

*/» 

=  about 

125°  or 

55°- 

TABLES  FOR  DETERMINING  MINERALS. 


147 


Polariza- 
tion- 
colors. 

Color  and 
power  of 
refracting 

light. 

Pleo- 

ehroisin. 

Structure. 

Associa- 
tion. 

Inclosures. 

Decom- 
position. 

Occurrence. 

Remarks. 

Inter- 

/3 =  1.639 

Very 

Partly  in 

With 

Inclosures 

Similar 

Like 

Can  be  dis- 

ference- 

Dark 

slight. 

large 

olivine, 

of  brown 

to 

enstatite, 

tinguished 

figures 

brown. 

irregular 

plagio- 

rectangular 

bastite, 

accessory 

from  : 

/ess 

grains  in 

clase, 

leaflets,  or 

into  a 

primary 

monoclinic 

brilliant 
by  far 

coarsely- 
granular 

mono- 
clinic 

opaque 
needles 

green 
fibrous 

constituent. 
Often 

augite  only 
by  examin- 

than in 
mono- 

rocks; 
partly  in 

augite, 
magnet- 

distributed 

||   00^00 

aggre- 
gate, with 

replacing 
monoclinic 

ing  the 
transverse 

clinic 

sharply- 

ite  ;  like 

(or,  after 

elimina- 

augite as 

sections  and 

augite. 

defined 

crystals  in 

enstatite. 

Tschermak, 
ii      />    \ 

II  oo./   ooj. 

tion  of 
Fe3O4 

essential 
constituent. 

the  exactly 
parallel 

the  por- 

Vitreous 

or 

Also  in  the 

direction  of 

phyritic 

inclosures. 

Fe203. 

younger 

extinction 

eruptive 

basic 

of  the  longi- 

rocks. 

eruptive 

tudinal 

rocks 

sections  ; 

and  the 

hyper- 

coarse- 

sthene by 

grained 

pleochroism 

older  ones. 

and  charac- 

ter of  the 

double- 

refraction; 

hornblende 

and  biotite 

by  the  want 

of  powerful 

pleochro- 

ism. 

Bril- 
liant. 

Dark 
brown. 

Strong 
pleo- 

Inclosures 
similar  to 

With 
olivine, 

Inclosures 
of  minute 

Very  rarely 
accessory 

Distin- 
guished 

ftp  =1.636. 
Relief 
not 
marked. 

chroism. 
Greenish 
yellow 
parallel 

bronzite. 
The  longi- 
tudinal 
sections 

plagio- 
clase, 
augite, 
and  horn- 

brown and 
greenish 
mica-like 
leaves, 

as 
secondary 
constituent. 
Decom- 

from : 
biotite  by 
cleavage, 
strength  of 

to  the 
striations 

generally 
fibrous  as 

blende. 

often 
regularly 

position- 
product  of 

pleo- 
chroism, 

(  II  ^). 
reddish 
brown 
at  right 
angles  to 

a  conse- 
quence of 
cleavage. 

arranged  ; 
otherwise 
very  poor 
in 
inclosures. 

olivine  in 
gabbro  and 
olivine- 
fels. 

and 
magnitude 
of  axial 
angle; 
hornblende 

them. 

Magnetite. 

by  the 

optical 

orientation  ; 

bronzite 

and 

hyper- 

stkene  by 

the  pleo- 

chroism 

(axial 

colors) 

and 

cleavage. 

148 


DETERMINATION   OF  ROCK-FORMING  MINERALS. 


ranee  of  the 


NAME. 

Chemical 
composition 
and 

reactions. 

Specific 
gravity. 

Cleavage. 

Ordinary 
combinations 
and  form 
of  the  cross- 
section. 

Twins. 

Optical 
orientation. 

Character 
and  strength 
of  double- 
refraction. 

Direc- 
tion of 
extinc- 
tion. 

1.  Hyper- 

sthene. 

Like 
bronzite, 
yet  far 

3-3-34. 
(3-34-) 

oo  P 
perfect. 

00/00 

Large 
irregular 
grains  and 

Knee-  A.P.  =  oo/co 
shaped  2.M.  -L  oP. 
twins    i.M.-Lco/'co 

\»P 

positive. 

||    00/00 

richer  in 
iron. 

con- 
choidal 

minute 
colu  tn  ns 

in  the 
crystals 

c'  =  C 

negative. 
Feebler 

separa- 
tion. 

00/00 

imper- 

$L 

about  92°. 

of 

00/>.  00/00 
00/00, 

also  :  £/oo 
2/00.3/3. 

as  in 
bronz- 
ite. 

a  =  0 
(See 
Fig.  5.) 
Large 
axial  angle. 
Dispersion 
about  a 

than  in  the 
monoclinic 
augites. 
(Compare 
bronzite.) 

P  ^  ?\ 

feeble. 

[According 

to  Tscher- 

mak  the 

acute  oo  P- 

angle  lies 

to  the  front. 

then 

A.P   =00/00 

c'  =  c. 

a  =  b.'] 

(See 

Fig.  74.) 

TABLES  FOR  DETERMINING   MINERALS. 
Second  Middle  Line  (-[-)  on  oP. 


149 


Polariza- 
tion- 
colors. 

Color 
and 

power  of 
refract- 
ing light 

Pleo- 
chroism. 

ritnicture. 

Association. 

Inclosures. 

Hccoin- 

powttoa. 

Occurrence. 

Remarks. 

Rather 
bril- 
liant. 
Com- 
pare 
bronz- 
ite. 

Light 
:o  dark 
arown. 
Black 
from 
ron  in- 
closures 

Strong, 
especially 
in  the 
ongitudi- 
nal 
sections 
and  the 

In  large 
irregular 
grains  in 
the  granu- 
lar older  •, 
and  in 
small, 

With 
plagioclase, 
olivine, 
and 
monoclinic 
augite. 

Numberless 
inclosures 
of  brown 
or  violet 
rectangular 
leaflets 
often  in  the 

Hyper- 
stheneoften 
decomposes 
into  a 
dirty- 
brown 
or 

In  grains 
in  gabbro, 
norites  in 
younger 
eruptive 
rocks  ; 
especially 

Distin- 
guished 
from  : 
bronzite  by 
the 
character 
of  the 

1.639. 

thicker 
sections. 
Axial 

augite-like 
crystals  in 
the 

large 
grains  with 
marked 

greenish 
fibrous 
aggregate 

in  augite- 
andesites, 
and  feld- 

double- 
refraction 
and  power- 

f.ll       .tlAXV 

colors: 

(l  "=• 

younger 
porphyritic 

il  00^0°, 

the  <r'-axis 

spathic 
basalts 

ul  pieo— 
chroism  ; 

hyacinth 
red,  b  = 

eruptive 
rocks. 

whereby 
they  appear 

and  similar 
to  that  of 

poor  in 
olivine.    As 

monoclinic 
a  ugite 

reddish 
brown. 

Primary 
constituent 

striated. 
(See 

enstatite; 
the 

primary 
essential 

often  only 
by  exami- 

c =  gray 
sh  green. 
Absorp- 
tion 

I.  O. 

Fig.  50.) 
Inclosures 
of  opaque 
needles 

decomposi- 
tion begins 
here  also, 
first  on  the 

constituent 
and 
together 
with 

nation 
with  the 
condenser, 
especially 

regularly 

fissures  and 

monoclinic 

in 

c'>d>~b. 

distributed 

especially 

augite. 

transverse 

often  occur 

on  those 

sections. 

in  the 

at  right 

and  by  the 

crystals. 

angles  to 

feebler 

Otherwise 

the  t'-axis. 

double- 

poor  in 

It  is  a 

refraction  ; 

inclosures. 

"  bast  it  e- 

biotite  by 

Vitreous 

like" 

absence 

particles. 

decom- 

of the 

position. 

marked 

cleavage  ; 
hornblende 

by  optical 

orientation 

and 

prismatic 

angle. 

ISO 


DETERMINATION  OF  ROCK-FORMING  MINERALS. 


NAME. 

Chemical 
composition 
and 
reactions. 

Specific 
gravity. 

Cleavage. 

Ordinary 
combina- 
tions and 
form  of  the 
cross-section. 

Twins. 

Optical 
orientation. 

Character 
and  strength 
of  double- 
refraction. 

Direction 
of 
extinction. 

2.  a.  Proto- 

3-054- 

bastite. 

(Dia- 

clasite.) 

In  sections 

\\oP 

Partly  in 

positive; 

rather 

conse- 

sharply- 
de  fined 

quently 
negative 

/3.  Bastite. 

Like 
bronzite. 
Contains 
HaO. 

2.6-2.8. 

00/00. 

oo  P. 

crystals, 
partly  in 
grains  of 
columnar 
form, 
and  in 
irregular 
leafy  in- 
dividuals. 
oo/>=93°. 

Very 
rare. 
Penetra- 
tion- 
twins. 

A.  P.  = 

II  oo  -Poo. 
2.  M.  -L  0p. 
I.  M.  -L 

00/00. 

c'  =  C 

Z=  a 
&  =  b 
Dispersion 

p   >    V 

about  a. 
Rather 
strongly 
doubly- 
refracting, 

like  hyper- 

• 

sthene. 

DISTINGUISHING  OF  THE  RHOMBIC  AUGITES 

The  three  rhombic  augites,  enstatite,  bronzite,  and  hypersthene,  are  in  general  distinguished 
only  by  the  amount  of  iron  present,  together  with  the  magnitude  of  the  optic  axial  angle  lying  in  the 
plane  II  oo/oo;  in  those  poor  in  iron,  i.  M.  (=  c'~)  -L  oP\  in  those  rich  in  iron  is  c'  to  2.  M.  (J-  oP}.  A 
positive  double-refraction  is  observable  in  the  transverse  sections  of  both  varieties,  only  the  magni- 
tude of  the  axial  angle  determining  whether  the  acute  angle  is  visible  ||  oP  or  ||  00/00. 

Pleochroism  also,  in  general,  allows  no  conclusion,  as  only  hypersthenes  which  are  very  rich  in 
iron  seem  to  show  the  mentioned  powerful  pleochroism.  Protobastite  and  its  decomposition  product, 
bastite,  however,  have  the  optic  axial  plane  II  oo/oo.  The  rhombic  augites  mentioned  above  also  show 
a  tendency  to  metamorphose  into  bastite. 

The  rhombic  augites,  as  regards  optical  orientation,  are  distinguished  from  the  monoclinic  by 
the  much  feebler  double-refraction  and  inferior  brilliancy  of  the  polarization-colors.  The  isotrope 


TABLES  FOR  DETERMINING  MINERALS. 


Polariza- 
tion- 
colors. 

Color  and 
power  of 
refracting 

light. 

Pleo- 
chroism. 

Structure. 

Associa- 
tion. 

Inclosures. 

Decom- 
position. 

Occurrence. 

Remarks. 

Rather 
brilliant. 

Light 
yellow- 

Proto- 
bastite, 

Into 
bastite. 

As 
primary 

Distinguish- 
able from 

ish. 

fresh  and 

essential  or 

enstatite  and 

free  from 

accessory 

bronzite 

inclosures. 

mineral  in 

only  by  the 

often 

certain 

optical 

shows  the 

gabbros 

orientation 

beginning 

and  por- 

(position  of 

of  a 

phyritic 

i.  M.). 

fibrous 
deconipcsi- 

augite- 
plagioclase 

tion  into 

rocks. 

bastite^  in 

Many 

that  the 

times  in- 

formation is 

With 

closures 

of  fibres 

plagio- 

like 

\\c'. 

clase, 

hyper- 

olivine, 

sthene 

mono- 

and 

Not  very 
brilliant. 

Dirty 
pale 
green. 

Very 

weak. 
Absorp 
tion 
c  >  a 

Commonly 
interpene- 
trated wiih 
olivine,  i.e., 
serpentine. 
A  metallic 

clinic 
augite, 
magnet- 
ite. 

bronzite. 
Inclos- 
ures of 
picotite 
and 
chromite. 

Bastite 
itself  is 
always  a 
decom- 
position- 
product 

As 

secondary 
decom- 
position- 
product  of 
rhombic 

Distinguished 
from  : 
serpentine 
by  the  stria- 
lion  parallel 
to  the  vertical 

lustre  on 

of 

augite  in 

axis  ; 

COA». 

rhombic 

olivine-fels, 

chlorite  by 

Finely 

augite. 

gabbro, 

less  perfect 

striated 

norite, 

cleavage,  and 

par  al  lei  to 
the  vertical 

andesites, 
rarely  in 

not  running 
II  oP,  by  the 

axis.   Often 

melaphyrs, 

feebler  pleo- 

shows  a 

and 

chroism,  and, 

remnant  of 

diabase- 

finally, 

fresh 

porphyries. 

almost  always 

enstatite  or 

by  the 

protobastite 
mineral. 

pseudo- 
morphs  after 

augite  crys- 

tals. 

FROM  EACH   OTHER  AND   FROM  THE   MONOCLINIC. 

sections  of  monoclinic  augite,  cut  at  right  angles  to  one  of  the  optic  axes,  show  two  to  three  rings  and 
clouds  ;  those  of  rhombic  augite  of  the  same  thickness,  none  or  at  most  one  ring. 

The  polarization  colors  of  rhombic  augite  in  very  thin  sections  are  generally  yellowish  white 
I.  O.,  II  0/>and  a>/~oo  (in  bronzite  and  hypersthene,  enstatite  shows  more  brilliant  polarization-colors) ; 
in  these  sections  also  a  biaxial  interference-figure  is  discernible  ;  if  monoclinic  augite,  a  blue  to  red, 
green,  and  side  appearance  of  one  of  the  optic  axes. 

Finally,  all  sections  of  rhombic  augites  parallel  to  the  c'-axis  have  a  parallel  extinction  i.  p.  p.  1.  ; 
of  monoclinic  augites,  an  extinction  with  a  varying  obliquity  (to  45°)  to  the  r-axis.  The  common  poly- 
synthetic  twins  of  monoclinic  augites  Hoodoo  are  wanting  in  the  rhombic  (seen  especially  well  on 
transverse  sections). 


152 


DETERMINATION  OF  ROCK-FORMING  MINERALS. 


y.  AXIAL  PICTURE  VISIBLE  IN  SECTIONS  ||  oP.; 
aa.   Appearance  of  the  I.  M. 


NAME. 

Chemical 
composition 
and 
reactions. 

Specific 

gravity. 

Cleavage. 

Ordinary 
combina- 
tions and 
form  of  the 
cross-section. 

Twins. 

Optical 
orientation. 

Character 
and  strength 
of  double- 
refraction. 

Direction 
of 
extinction. 

1  a.  Anda- 
lusite. 

AlaSiOB. 
Not  acted 
on  by  acids. 

3.10- 
3.17. 

Prismatic 

Imperfect 
after 

00  /^OO, 

Rarely 
grains; 
long 
columns 

OOP.0/J.POO. 

. 

c»  /->»; 

Quadratic 

and 

transverse 

Poo. 

sections, 

COP  = 

90°  50'. 
Separa- 
tion 

longi- 
tudinal 
sections, 
elongated 

\oP. 

rectangles. 

A.P.   ||   00/00 

i.M.  -LoP. 

Double- 

c'  =  0 

refraction 

1  =  b 

strongly 

j3.  Variety 
of  anda- 
lusite: 

a  =  C. 
Large 
axial  angle. 

negative. 

Ohiastolite. 

ditto. 

2.9-3.1. 

Perfect 

00  P. 

Long 
columns 

COP  = 

<xP.oP; 

91°  40'. 

not  in  form 

of 

microlites. 

TABLES  FOR  DETERMINING  MINERALS. 


153 


IN  THESE  DOUBLE-REFRACTION  NEGATIVE. 
on  oP  Negative. 


Polariza- 
tion- 
colors. 

Color  and 
power  of 
•efracting 

light. 

Pleo- 
hroism. 

Structure. 

Associa- 
tion. 

nclosures. 

Decompo- 
sition. 

Occurrence. 

Remarks. 

Very 

bril- 

Colorless ; 
tinged 

Very 
strong; 

Rarely  in 
grains,  almost 

With 
quartz, 

Some- 
times 

Com- 
monly 

As  primary 
accessory 

Distinguished 
from:  colorless 

liant. 

flesh-red. 

a  = 

always  in 

ortho- 

very  poor, 

decom- 

constituent 

augite  by 

Relief 

dark 

columnar 

el  ase, 

and  again 

posed 

in  granite 

he  commonly  oc- 

very 
marked. 

blood- 
red, 

individuals. 
Like 

biotite, 
and 

in  the 
meta- 

into  a 
greenish 

and  in 
crystalline 

curring  pleochro- 
ism  (reddish 

0p  =1.638. 

)  =  oil- 
green, 

staurolite, 
often  so  filled 

mus- 
covite. 

morphic 
schists 

fibrous 
aggregate. 

schists,  as 
mica  schist, 

green),  and  by  the 
always  parallel 

c  = 
olive- 

with 
Inclosures 

very  rich 
granules, 

which  has 
a  certain 

granulite. 
As  meta- 

extinction;   ensta- 
ite  by  the  charac- 

green. 

absorp- 
tion 

hat  but  little 
of  the 
andalusite 
substance  is 

in 
quartz 
Ejranules, 
bitumi- 

similarity 
to  the 
decom- 
position- 

m  orphic 
mineral 
in  the 
contact- 

er  of  the  double- 
refraction;  hyper- 
sthene  by  the  color 
and  character  of 

to  be  seen. 

nous  in- 

product 

schists. 

double  refraction; 

'>  t>>  a. 

Often  in  long 

closures, 

of  cor- 

hornfels. 

".oisite  by  the  pleo- 

thin  needles 

and 

dierite. 

chroism,  form  of 

in  radial 

leaves  of 

cross-section,  pris- 

aggregates. 

biotite. 

matic  angle,  and 

position  of  the 

i  .  M  .  ;   sillima  n  ite 

only  with  diffi- 

culty if  occurring 

n  minute  needles. 

Sillimanite  gen- 

erally occurs  in 

minute  needles, 

andalusite  in  large 

columns  or  grains. 

They  differ,  more- 

over, in  pleochro- 

ism,  prismatic 

angle,  and 

cleavage. 

The  trans- 

Bitumen. 

Similar  to 

Rarely  in 

Characterized 

verse  sections 

Leaves 

anda- 

in e  ta- 

by the  regular 

present  a 
peculiar 

of  mus- 
covite 

lusite; 
whole 

in  or  ph  ic 
schists 

inclosures. 

structure. 

and 

pseudo- 

(contact  on 

Quadratic 

biotite. 

morphsof 

granite). 

cores, 

quartz, 

nclosuresof  a 

mus- 

bituminous 

covite, 

substance,  are 

and 

arranged 

chloritic 

at  the  centre 

substance 

and  on  the 

after 

four  corners; 

chiasto- 

very  common 

lite. 

are  the 

remarkable 

regularly- 

disposed 

inclosures  of 

carbonaceous 

particles. 

(See  Fig.  76.) 

154 


DETERMINATION  OF  ROCK-FORMING  MINERALS. 


NAME. 

Chemical 
composition 
and 
reactions. 

Specific 
gravity. 

Cleavage. 

Ordinary 
combina- 
tions and 
form  of  the 
cross-section. 

Twins. 

Optical 
orientation. 

Character 
and  strength 
of  double- 
refraction. 

Direction 
of 
extinction. 

Cordierite. 
(Dichro- 
ite.) 

Mg2R2 

2.59-2.66. 

oo/oo, 
imperfect 
Pco. 

In  large 
grains  and 
small 

If  in 
crystals 
so  often 

AW/ool! 
i.  M.  -L  oP. 

Negative, 
not  very 
energetic. 

A122  Fe2. 
Scarcely 

119°  io~'. 

crystals. 

00  P.    00/00. 

pene- 
tration 

c'  =  a 

b    =  C 

attacked 

oP. 

twins 

a,  =  b 

by  acids. 

Hexagonal 
transverse 

and 
four- 

Axial  angle 
rather 

sections 
and  rect- 
angular 

lings 
after 
00  PI 

large. 
Dispersion 
feeble 

longitu- 

rarely 

p  <  v. 

dinal 

after 

sections. 

00/3. 

(See 

Figs. 

23  and 

78.) 

Finite 

Large 

(Decom- 

crystals = 

position- 

00  />.   00/00. 

product 

oo/oo  .  0P. 

of  cor- 

dierite.) 

bb.  Appearance  of  2  M. 


Zoizite. 

H2Ca4 

s'£t 

Attacked 
by  acids 
only  with 
difficulty; 
after 
ignition, 
however, 

3.22-3.36. 

00/00 

very 
perfect. 
Separa- 
tion 
II  oP. 

Elongated 
grains  and 
long 
transverse- 
limbed 
columns. 

<X>P  .    00/00 

(see  Fig. 
79). 
Hexagonal 

A.  P.  II 

00/00 

always 
«  =  c    = 

I  =  b 
c'  =  a. 
Or  if 

A.  P.  |i  oP, 

InoP 
appearance 
of  2. 
negative  M. 
positive. 
Feeble 
double- 
refraction. 

soluble 

transverse 

7,  —  n 

with 

sections. 

V     —    U 

c'  —  b' 

separation 
of 
amorphous 
Si02. 

<X>P  = 

116°  26'. 

L      —    U, 

very 
powerful 
dispersion 
p  <  v. 

About  a! 

(if  A.  P. 

1!  oP,  then 

dispersion 

• 

p   >    7/.) 

TABLES  FOR  DETERMINING  MINERALS. 


155 


Polari- 
zation- 
colors. 

Color  and 
power  of 
refracting 

light. 

Pleo- 

ehroism. 

Structure. 

Associa- 
tion. 

Inclos- 
ures. 

Decomposi- 
tion. 

Occurrence. 

Remarks. 

Rather 
bril- 

Violet- 
blue. 

marked. 

Always  in 
larger 

With 
quartz. 

Fluid 
inclos- 

Very 
common, 

Rare,  as 
accessory 

In  thin  sections 
and  in  grains 

liant, 
like 

In  very 
thin 

In  thicker 
sections 

individuals; 
never  in 

ortho- 
clase, 

ures, 

silli- 

especially 
if  occurring 

primary  con- 
stituent in 

often  very 
similar  to 

quartz. 

sections, 

well 

microlites. 

and 

manite 

in  grains  or 

granite, 

quartz,  yet 

colorless. 
/3=  1.54- 

recog- 
nizable. 

In  rounded 
larger 

biotite. 
With 

needles, 
pleo- 

large  crys- 
tals, on  the 

quartz-por- 
phyry (pinite). 

easily  distin- 
guished by  the 

1.56. 

a  =  yel- 
lowish . 
white. 

grains,  or 
in  small 

crystals  ; 

plagio- 
clase, 
quartz, 

naste 
crystals, 
zircon. 

crevices,  or 
completely 
decomposed 

and  in  grains 
in  gneiss. 
Rarely  in 

phenomena  of 
decomposition 
on  the  crevices 

b  =  pale 

the  latter  in 

sani- 

vitreous 

(pinite)  into 

crystals  in 

in  c.  p.  1. 

to  prns- 

eruptive 

dine. 

inclos- 

a  greenish 

tracliytes 

If  in  crystals, 

sian  blue. 

rocks. 

augite, 

ures. 

fibrous 

(twins  !),  and 

recognized  by 

c  =  dark 

Meta- 

pleo- 

aggregate, 

in  the 

the  color  and 

Prussian 

morphic 

naste, 

similar  to 

trachytic 

pleochroism. 

blue 

mineral. 

corun- 

andalusite. 

volcanic 

Absorp- 

dum. 

(See  Fig. 

overflows. 

tion 

77.) 

Aggre- 

Green. 

7>>  d>c'. 

Wholly 

With 

Easily  recog- 

gate 

Colorless. 

composed 

quartz, 

nized  macro- 

polari- 
zation. 

of  minute 
threads  and 

ortho- 
clase, 

scopically  by 
the  crystalline 

leaflets. 

and 

•orm,  and  from 

biotite. 

the  decom- 

position. 

(negative)  on  oP. 


Gene-  1  Colorless 

The 

With 

Fluid 

Often 

Common  in 

Distinguished 

rally 
feebly 
bril- 

— white. 
/3p  =  1.70. 
Relief 

transverse 
crevices  on 
the  long 

quartz, 
ompha- 
cite, 

inclos- 
ures 
are 

opaque  on 
the  border. 

crystalline 
schists  as 
eclogites  and 

from: 
apatite  easily 
by  the  optical 

liant 
blue- 
green. 

very 
marked. 

columns 
and  the 
inclosures 

garnet, 
mica, 
horn- 

very 
com- 
mon. 

especially 
amphibolites. 

properties  ; 
andalusite  by 
the  cleavage  in 

are  charac- 

blende. 

sections  parallel 

teristic. 

oP^  moreover 

(See  Fig. 
79.) 

by  the  pleochro- 
ism and  polari- 

zation-colors ; 

sillitnanite  by 

the  polarization- 

colors  and  the 

optical  orienta- 

tion (never 

sinks,  like  silli- 

manite,  to  the 

microliticform); 

olivine  by  the 

crystalline  form 

and  polariza- 

tion-colors 

(optical  investi- 

gation, power  of 

dispersion); 
etistatiteby  the 

optical  orienta- 

tion, the  polari- 

zation-colors, 

cleavage,  and 

the  oo  /"-angle. 

156 


DETERMINATION  OF  ROCK-FORMING  MINERALS. 


II.  b.  2.  Minerals  Crystallizing 

a.  MINERALS  APPARENTLY  CRYSTALLIZING 

CLEAVAGE  MOST 


NAME. 

Chemical 
composition 
and 
reactions. 

Specific 
gravity. 

Cleavage. 

Ordinary 
combina- 
tions and 
form  of  the 
cross-section. 

Twins. 

Optical 
orientation. 

Character 
and  strength 
of  double- 
refraction. 

Direction 
of 
extinction. 

1.  Mica 

Group. 

1 

a.  Mer 

oxene 
(Biotite). 

m  R4SiO4 

n 

2.8-3.2. 

Highly 
eminent 

as  P  quite 

120°. 

Rare. 
Twin- 

A.  P.  ||  cc  Poo 
(mica 

As 
hexagonal 

The 
trans- 

n R2SiO4 

\«P. 

OOP.    00  POO    . 

n  ing- 

second 

as  a  conse- 

verse 

VI 

Separa- 

oP.     Thin 

plane, 

class). 

quence 

sections 

v  R2Si3012 

tions 
corre- 

tablets or 
short 

ooP; 
both  in- 

A. P.  paral- 
lel to  two 

of  the 
apparently 

generally 
apparent- 

R =  K,  Na. 

sponding 

columns. 

dividuals, 

opposite 

constant 

ly 

H, 

to  the      Transverse 

however, 

sides(coPoo) 

parallel 

isotrope; 

n 

pressure-      sections 

forced 

of  a 

extinction 

the  lon- 

R= Fe, 

surfaces         (  H  oP) 

over  each 

hexagon 

and  of  the 

gitudinal 

Mg, 

or 

hexagonal 

other  in  a 

and 

small 

sections 

R2  =  A12, 
Fe2. 

Slightly 

"sliding- 
planes" 
(gleit- 

tablets 
without 
cleavage- 

plane 
quite  H  oP\ 
also  with 

coinciding 
with  a 
u  fracture- 

axial  angle, 
or,  as  the 
i.  M.  differs 

with 
parallel 
extinc- 

attacked by 

flache) 

cracks. 

several 

line" 

but  little 

tion, 

HC1, 

-  P3  and 

More  often 

lamellae 

(schlag- 

from  the 

therefore 

but 

\P&>. 

with 

inter- 

linie), 

normal 

cannot 

completely 

"sliding 

polated. 

i.M    =a 

to  0P, 

be 

decom- 
posed by 

plane" 
(gleit- 

Only  the 
latter 

vary  but 
little  from 

apparently 
rhombic. 

studied  in 
c.  p.  1. 

H2S04 

fiache), 

recogniz- 

the normal 

Negative. 

Ap- 

with 

three  line- 

able 

to  oP 

parently 

separation 

systems 

i.  p.  1. 

Axial 

hexago- 

ot a  silica- 

crossing 

angle 

nal. 

skeleton. 

each  other 
at  an 

generally 
very  small 

angle  of 

—  -  5°~*3° 

60°,  and 

but 

rectangular 

variable, 

longitudi- 

being 

nal 

sometimes 

sections 

very  large. 

(  ||  to  the 

Dispersion 

<r'-axis), 

p  <  v. 

with 

(See 

numberless 

Fig.  19.) 

cleavage- 

lines 

parallel  to 

According 

the  longer 
sides. 

to 
Tschermak 

(See 
Figs.  80 

mixtures 
of 

and  8  1.) 

3si636242  3 

and 

The  axial 

Mg12Si6024 

angle 

in 

lessens  with 

proportion 

decrease 

of 

of  iron 

i  :  i  or  2  :  i. 

present. 

TABLES  FOR  DETERMINING  MINER  A^ 
in  the  Monoclinic  System. 

IN   THE  HEXAGONAL   (OR  RHOMBIC)   SYSTEM. 

PERFECT  ||  oP. 


,., 


'57 


Polariza- 
tion- 
colors. 

Color  and 
power  of 
refracting 
light. 

Pleochroism. 

Structure. 

Associa- 
tion. 

Inclosures. 

Decomposi- 
tion. 

Occur- 
rence. 

Remarks. 

Not 
particu- 
larly 

Brown- 
black, 
dark 

Transverse 
sections 
show 

Primary 
constituent 
I.  0. 

Gen- 
erally 
with 

Generally 
free  from 
inclosures; 

Into 
chloritic 
minerals 

In  nearly 
all  rocks. 
In  many 

Easily  re- 
cognizable 
by  eminent 

brilliant, 
brownish 
tints  ; 
in  very- 
thin 

green. 
JB  =  1.61. 

almost  no 
pleochro- 
ism. 
In  longi- 
tudinal 

Partly  in 
large 
crystals 
(in  eruptive 
rocks) 

quartz 
and 
ortho- 
clase  ; 
horn- 

yet often 
numbers 
of  epidote 
needles 
arranged 

•very 
commonly 
-with 
epidote  and 
calcite. 

as 
essential 
primary 
constitu- 
ent. 

cleavage 
and  exceed- 
ingly 
powerful 
pleochro- 

leaflets 

sections 

often 

blende, 

in  tufts 

In  this 

One 

ism.     Dis- 

irides- 
cent, 
carmine- 
red. 

very 
strong, 
stronger 
than 

cracked 
or 
shattered, 
or  with 

more 
rarely 
augite 
and 

(comp. 
decomposi- 
tion), 
or  long 

decompo- 
sition the 
biotite 
loses  its 

of  the 
first- 
formed  of 
minerals. 

tinguished 
from  : 
hornblende 
in  that  the 

hornblende. 

broad 

olivine. 

thin  rutile 

brown 

As  decom- 

transverse 

a  and  b 

opaque 

needles 

color,  and 

position- 

sections 

differing 
but 

border; 
or  in 

arranged 
very 

becomes 
green; 

product 
of  augite. 

are  not 
pleochroitic 

slightly. 
In 

minute 
irregular 

regularly 

at  60°; 

lenticular 
calcite 

horn- 
blende, 

(and  by  the 
investiga- 

longitudi- 

leaflets, 

apatite, 

separates 

rarely  of 

tion 

**      . 
nal 
sections 

especially 

in  the 

zircon. 

between 
the  leaves, 

olivine. 
As 

i.  c.p.  1.)— 
the  longi- 

parallel 

crystalline 

and 

contact- 

tudinal 

to  the 

schists  ; 

needles  of 

mineral 

sections 

c'-axis 

or  dis- 

epidote 

in  meta- 

can  be 

(=  the 

tributed 

appear. 

morphic 

distin- 

shorter 

through  the 

(See 

schists. 

guished 

sides  of 

ground- 

Figs.  80 

by  the. 

the 

mass 

and  81.) 

oblique 

rectangle), 
a  =  yellow 

constituent 
II.  O., 

In  the 
decomposi- 

extinction 
to  be 

pale 

as  in  the 

tion  of 

proven  on 

brown; 

Dasalts  and 

biotite  also 

horn- 

perpen- 

other rocks. 

a  separa- 

blende; 

dicular  to 

tion  of 

chlorite 

the  c'-axis 

ferric 

in  that 

(parallel 

hydroxide 

chlorite  is 

to  the 

or  magnet- 

always 

longer 

ite  about 

green, 

sides). 

the 

never  so 

c  =  dark 

periphery. 

well 

broivn  to 

crystal- 

black. 

lized, 

Absorption 
c  >  b  >  a. 

feebler 
pleochro- 

itic, and 

generally 

arranged 

in  tufts. 

DETERMINATION  OF  ROCK-FORMING  MINERALS. 


NAME. 

Chemical 
composition 
and 
reactions. 

Specific 
gravity. 

Cleavage. 

Ordinary 
combina- 
tions and 
form  of  the 

cross-section. 

Twins. 

Optical 
orientation. 

Character 
and  strength 
of  double- 
refraction. 

Direction 
of 
extinction. 

b.  Rubellan. 

Like 

See 

See  mer- 

Large 

Large 

See  meroxene. 

biotite 
(meroxene); 

mer- 
oxene. 

oxene. 

thin 
hexagonal 

axial  angle. 

rich  in 

tables. 

iron. 

c.  Phlogo- 
pite. 

A 
magnesian 
mica, 
nearly  free 
from  iron. 
According 
to 
Tschermak, 
a  mixture 
of 
K6(A12)3 
Si6O24, 
H8S,10024. 

2-75- 
2.97. 

See  mer- 
oxene. 

See 
meroxene. 

See  mer- 
oxene; 
also  twins 
after  oo  P 
with  indi- 
viduals 
lying 
near  one 
another. 

A.P.  n  oopoo 

a  nearly 
J-oP. 
c-.Q.  =  -2\°. 
Dispersion 

Axial  angle 
about  15°. 

Negative 
Hke 
meroxene. 

Like 
mer- 
oxene, 
alwa;  s 
parallel 
extinc- 
tion. 

and 

Mg12Si6Oa4 

in  the 

proportion 

of  nearly 

3:1:4- 

d.  Anomite. 

According 
to 
Tschermak, 

See  mer- 
oxene; 
also  here 

See 
meroxene. 

A.P.j.00^00 
(mica 
I.  class). 

Negative. 

ditto. 

a  mixture 

the 

o  nearly 

of 

"  gliding- 

\oP. 

H2K4(A12)3 

planes" 

Axial  angle 

Si6O24 

very  com- 

about 

and 

monly 

12-16° 

Mg12Si6024 
in 

dis- 
cernible. 

and  less. 
Dispersion 

proportion 

One  of 

p>   V. 

of  i  :  i 

the 

or  2  :  i. 

gliding- 

planes 

parallel 

TABLES  FOR  DETERMINING  MINERALS. 


159 


Color  and 

Polariza- 
tion-colors. 

power  of 
refracting 

Pleo- 
chroism. 

Structure. 

Associa- 
tion. 

Inclosures. 

Decomposi- 
tion. 

Occur- 
rence. 

Remarks. 

light. 

Brown- 
ish, red, 
brick-red. 

Often 
appears  as 
a  foreign 
inclosure; 

With 
augite, 
olivme 
plagio- 

Augitic 
needles, 
ferric 
hydrate, 

Depositing 
ferric 
hydroxide. 

In 
basalts 
and 
lavas. 

Dis- 
tinguished 
from 
biotite 

only 
primary 

clase, 
nephe- 

and 
microlites 

only  by  the 
color. 

constituent 
1.0.    Is 

line,  or 
leucite. 

regularly 
arranged 

only  an 

at  60° 

Altered 

as  in 

(pyrogene?) 
biotite. 

meroxene. 

See  mer- 
oxene. 

Yellow, 
pale 
brown, 
red- 
brown, 
like  mer- 

Very 
strong, 
yet 
weaker 
than 
mer- 

Mostly 
in  thin 
irregular 
leaflets. 

With 
calcite 
and 
ser- 
pentine. 

Very  poor; 
as  in 
rubellan, 
regular 
layers  of 
thread-like 

Becoming 
green, 
like 
meroxene. 

In 

granular 
rarely 
compact 
lime- 
stones^ 

Differs 
from 
meroxene 
only  in 
chemical 
composition 

oxene. 

oxene. 

needles. 

dolo- 

and color. 

Relief 

mites, 

marked. 

and  in 

serpen- 

tine 

rocks. 

ditto. 

Red- 
brown. 

ditto. 

ditto. 

With 

olivine, 
augite, 
and 
actino- 

lite. 

Becoming 
green  to 
colorless 
as  above. 
At  the 
beginning 
of  the 
decomposi- 
tion it 
becomes 

Rare  in 

olivine- 
fels. 

opaque,  and 

contains 

numbers 

of  brown 

grains 

inclosed. 

i6o 


DETERMINATION   OF  ROCK-FORMING  MINERALS. 


NAME. 

Chemical 
composition 
and 

reactions. 

Specific 
gravity. 

Cleavage. 

Ordinary 
combina- 
tions and 
form  of  the 
cross-section. 

Twins. 

Optical 
orientation. 

Character 
and  strength 

of  double- 
refraction. 

Direction 

of 

extinction. 

e.  Musco 

vite 
(and 
sericite). 

H4K2(A12)3 
Si.OM. 

Not 
attacked  by 
acids. 

2.76-3.1. 

Very 
perfect 
II  oP: 
''  sliding- 
planes" 

Rarely 
crystallized 
in  rocks; 
hexagonal 
tables. 

See 
merox- 
ene. 

A.P.j_oojPoo 

(mica  I. 
class), 
a  differing 
but  little 

Strongly 
negative. 

Like 
magnesia- 
mica 
with 
parallel 

(gleit- 

from  c' 

extinc- 

flache) 
as  in 

nearlyj.o/'. 
Dispersion 

tion  ;  ap- 
parently 

mer- 

p  >    V. 

rhombic. 

oxene. 

Axial  angle 

generally 

large, 

60-70°. 

(See 

Fig.  18.) 

2.  Taic. 

HjMgs 
Su013. 
Not 
attacked  by 
acids. 
Al- 
reaction. 

2.69-2.8. 

Eminent 
\oP 

(imper- 
fect oo/>). 

Never  in 
crystals; 
in  rocks 
mostly  in 
minute 
irregular 
leaflets  like 
mica. 

A.  P.  ||  oo  Poo 

II  to  a 
fracture- 
line 
(schlag- 
linie). 
a  nearly 
JL0P. 
(According 
to 
Tschermak 
axial  angle 
about  17°.) 

Feebly 
negative. 

Apr 

parently 
rhombic  ? 

TABLES  FOR  DETERMINING  MINERALS. 


161 


Color  and 

tion- 
colors. 

power  of 
refracting 
light. 

Pleo- 
chroism 

Structure. 

Association. 

Inclosures. 

Decom- 
position 

Occurrence. 

Remarks. 

Exceed- 
ingly 

brilliant, 

Colorless, 
light 
green, 

As 
primary 
constituent 

With 
quartz, 
orthoclase, 

Very  poor; 
rarely 
rutile 

As 

primary 
constituent 

Easily  recog- 
nizable by 
the  highly 

irides- 

oil-green 

I.  O.  in 

biotite. 

needles, 

in  granites, 

eminent 

cent 
(red  to 
yellow 
colors). 

large 
leaves  and 
tables,  in 
tufted  and 

tourmaline 

hematite 
tablets,  or 
tourmaline 
columns. 

especially 
tourmaline 
granites, 
and  in 

cleavage  and 
brilliant  po- 
larization-col- 
ors; yet  diffi- 

stellate 

Zircon. 

crystalline 

cult  to  distin- 

aggregates. 

schists: 

guish  with  the 

As 

especially 

microscope 

secondary 

prominent 

from  talc. 

* 

product 

in  gneiss, 

Sericite  is 

in 

mica-schist, 

only  a  musco- 

aggregates 
of  minute 

and  clay 
schists. 

vite,  appear- 
ing like  talc, 

irregular 

As  primary 

soft, 

leaflets. 

constituent 

greasy  to  the 

In 

nowhere 

touch,  non- 

crystalline 

else  in 

elastic  and 

schists  in 

eruptive 

occurring  in 

minute 

rocks.    As 

compact 

irregular 

decomposi- 

aggregates of 

leaflets. 

tion-product 

minute  irreg- 

in the 

ular  leaflets, 

feldspars, 

in  certain 

chiastolite, 

semi-crystal- 

iebenerite, 

line  schists. 

etc. 

See 
musco- 
vite. 

Colorless, 
white, 
light 
green. 

Mostly  in 
'rregularly 
disposed 
interlaced 

With 
quartz, 
orthoclase, 
mica, 

Very  poor. 
Biotite, 
actinolite. 
Like 

As 
primary 
constituent 
in  many 

Difference  be- 
tween musco- 
vite and  talc  : 
Muscovite  oc- 

or 

or  with 

muscovite. 

crystalline 

curs  general- 

rosette- 

augite  and 

schists. 

y  in  large  in- 

sh aped 

olivine. 

Not 

dividuals 

stellate 

common. 

remarkable 

aggregates 

As  second- 

for the  basal 

of  minute 

ary  product 

cleavage,  or 

,- 

leaflets. 

in  the  de- 

in separate 

composition 

leaves. 

of  augites 

Talc,  how- 

and horn- 

ever, occurs 

blendes 

generally  in 

poor  in 

aggregates  of 

iron, 
especially 

compact  inter- 
twined mi- 

enstatite 

nute  leaflets 

before  oc- 

arranged in 

curring  in 

stellate 

olivine-fels 
and 

groups. 
The  micro- 

serpentines 

chemical  in- 

vestigation of 

isolated  leaf- 

lets with  hy- 

drofluo-silicic 

* 

acid  is  the 

only  safe  one. 

1 62 


DETERMINATION  OF  ROCK-FORMING  MINERALS. 


NAME. 

Chemical 
composition 
and 

reactions. 

Specific 
gravity. 

Cleavage. 

Ordinary 
combina- 
tions nnd 
form  of  the 
cross-section. 

Twins. 

Character 

oiSffiSL  stl;Sihe-of 

refraction. 

Direction 
of 
extinction. 

3.  Chlonte 

Group. 

«.  Ripido- 
lite. 
(Chlorite  in 
restricted 
sense.) 

Mixture  oj 
/<2H20. 
3MgO. 
2Si02)  + 
q  (2H20  . 
2MgO  . 
A12O3  . 

2.78- 
2-95- 

Very 
perfect 
\\oP. 

Leaflets 
and  six- 
sided 
tablets 
ooP.  oP 
like  hexag- 
onal.    If 

Apparently 
hexagonal 
(optically- 
uniaxial), 
often 
clearly 
optically- 

Feebly 
negative. 

Cleavage- 
leaflets 
like 
isotrope. 
Longi- 
tudinal 
sections 

Si02). 

monoclmic, 

biaxial, 

with 

p  :q  —  t  :  2. 

then  ooP. 

wit  ft  very 

parallel 

oo  Poo  .  oP. 

small  axial 

extinc- 

b. Hel- 

Decom- 

ditto. 

Long  ver- 

Six-sided 

a  ngle. 

tion. 

minth. 

posed  by 

ftiicttlar 

leaflets 

a  J-oP. 

H2SO4. 

curled 

with  re- 

columns. 

entrant 

angles. 

c.  Pennin- 
ite. 

See  a. 
f.f  =  3:2. 
Decom- 
posed by 
HC1. 

2.61- 
2.77. 

duto. 

Crystals 
like 
rhombo- 
hedra  oP. 
R  or  3/? 

Pene- 
tration 

tkree- 
lings. 
(Biaxial 

Often 
clearly 
optically- 
biaxial. 

Sometimes 
positive, 
sometimes 
negative  ; 
very  feeble. 

Longi- 
tudinal 
sections 
with 
parallel 

or  oo^?  .  R. 

parts. 

extinc- 

Visible in 

tion. 

three 

Cleavage- 

d.  Kaem- 
mererite. 

Contains 
Cr203. 

2.617- 
2.76. 

ditto. 

Irregular 
leaflets 
apparently 
P  .  oP. 

positions 
differing 
by  120° 
in  leaves 

Clearly 
optically- 
biaxial. 

leaflets 
some- 
times 
isotrope, 

II  oP.) 

some- 

times 

double- 

refract- 

ing. 

e.  Clino- 
chlore. 

/:*=2:3. 

More 

2.65- 
2.78. 

ditto. 
"  Sliding 

Crystals  of 
motioclinic 

Com- 
monly in 

A.  P.  ||  co  Poo, 
often  also 

Generally 
positive. 

C  :  c  = 
12-15°. 

difficultly 

planes" 

habit 

twins  and 

_L  ooPoo; 

decom- 

(gleit- 

&P  .  ooPoo. 

three- 

C  quite-i-^f. 

posed  by 

flache) 

oP.  etc. 

lings. 

Varying 

acids  than 
the  above. 

similar 
to  mica. 

oo  P  quite 
120°.     In 

Twinning 
plane  a 

about  12-15° 
from  the 

large 

face  of 

normal  to 

leaves. 

the  hemi- 

oP.    Large 

pvramid 

axial  angle. 

"  M>. 

Dispersion 

P    <    "!'. 

/.  Chlori- 

toid 

H2R(AI2) 
Si07; 

3.52- 
3.56. 

II  oP. 
Not  so 

ditto. 
Tablets. 

Common- 
ly tablets 

A.  P. 
||    oo  Poo. 

Negative. 

a  :  c  — 
5-12°. 

R  =  FeO 

perfect 

of  thin 

i.  M.  differs 

and 

and  some 

as  in  the 

eaves  de- 

about 12° 

MgO. 

others. 

veloped 

from  the 

Decom- 

twin-like. 

-LoP. 

g.  Sismon- 

posed  by 

which  are 

Feeble. 

dine. 

concen- 

placed at 

trated 

120°  to 

H2S04. 

each 

other. 

TABLES  FOR  DETERMINING  MINERALS. 


Color  and 

Polari- 

zation- 
colors. 

POWIT  (if 

ivf  nil-tin}, 
light. 

Pleo- 
chroism. 

Structure. 

Assucia 
tion. 

IncJos- 

mvs. 

Decora 
position 

Occi  rrence. 

Remarks. 

Primary. 
More  com- 

Feebly 
bril- 

Light 
to  dark 

Very 
feeble. 

The  chlorites         As 
do  not  for  the    primary 

Very 
poor. 

monly  in 
leaflets  in 

liant. 

green. 

most  part 

con- 

Hema- 

chloritic 

bluish, 

M  =  1-575 

occur  as  rock- 

stituent 

tite  and 

schists,  as  de- 

blue  to 
green. 

constituents 
in  large  lamel- 

with 
quartz, 

hydrated 
ferric 

comppsition- 
product  of 

lary  hexag- 

ortho- 

oxide, 

mica,  augite, 

onal  tablets 

clase, 

and 

hornblende. 

ike  the  micas, 

biotite, 

needles  ol 

and  garnet. 

Difficult  to 

but  like  talc 
in  aggregates 
of  tm  nut  e  ir- 
regular leaf- 
lets either 

and 
musco- 
vite. 

i  utile  and 
actinolite. 

As  decomposi- 
tion-product 
aftermicaand 
hornblende, 
and  inter- 

distinguish 
from 
decomposed 
or  green- 
colored   mica. 

singly  or 
disposed  in 
radial  groups. 

penetrated  in 
minerals  of 
the  crystal- 
line schists. 

The  chlorites 
as  rock- 
constituents 
are  extremely 

Rare. 

difficult  to 

See 
ripido- 

Leek  to 

bluish 

Feeble. 
Green 

ditto. 

ditto. 

Rare  as  rock- 
constituent 

d  isti  nguish 
from  each 

lite. 

green. 

shades. 

as  above,  in 

Clinochlore 

leaves. 

alone  (also 

ottrelite)  is 

well  charac- 

terized 

through  the 

Peach  to 

Often  inter- 

With 

By 

Rarely  in 

pronounced 

blood-red. 

penetrated 

olivine, 

decom- 

serpentines. 

pleochroism 

with  clino- 

augite, 

position 

as  well  as  the 

chlore. 

and 

is  de- 

common 

chromite. 

color- 

twinning; 

zedand 

more  easily 

resem- 

determined 

bles 

by  optical 

talc. 

examination. 

More 
brilliant 
than  in 
the 

Dark  oil 
to  bluish 
green. 

Often 
very 
strong.  In 
sections 

In  larger 
leaves,  yet 
not  so 
marked  by 

With 
quartz, 
ortho- 
clase,  and 

ditto. 

Primary. 
Common  in 
crystalline 
schists,  as 

Ottrelite  is 
marked  by 
the  greater 
lardness,  less 

other 

-LoP 

lamellae  as 

mica. 

chloritic 

perfect 

chlo- 
rites. 
Indigo- 
yellow. 

yellow  ; 
II  c  light 
green, 
yellowish 
green. 

mica. 

With 
augite, 
horn- 
blende, 
olivine, 

schist,  and 
secondary  in 
serpentine. 

cleavage, 
absence  of 
laminations, 
and  richness 
of  inclosures  ; 

-L  c  blue- 

or  ser- 

also dis- 

green, 
dark- 

pentine. 

inguished  by 
chemical 

green. 

quantitative 

See 

Dark 

See 

ditto. 

With 

Fluid  in- 

Chloritoid  in 

ana  ysis. 

clino- 

green. 

clino- 

quartz, 

closures 

certain  semi- 

chlore. 

chlore. 

ortho- 

very 

crystalline 

\\c 

clase,  and 

common. 

schists. 

yellowish 

mica. 

Rutile 

Sismondine 

green. 

With 

needles. 

rarely  in 

-Lr 

augiie, 

glaucophane- 

greenish 

rutile, 

eclogite. 

blue. 

titanite, 

glauco- 

phane. 

164 


DETERMINATION  OF  ROCK-FORMING  MINERALS. 


NAME. 

Chemical 
composition 

and 

reactions. 

Specific 
gravity. 

Cleavage. 

Ordinary 
combina- 
tions and 
form  of  the 
cross-section. 

Twins. 

Optical 
orientation. 

Character 
and 
strength  of 
douijie- 
ref  faction. 

Direction 
of 
extinction. 

k.  Ottrelite. 

H6R3(A12)2 
Si6024. 
R  =  Fe,Mn. 

4-4(?)- 

oP  very 
perfect. 
Besides, 

Thin 
spherical 
tablets; 

See 
sismondine; 
poly 

Optically 
biaxial  ; 
i.M.  rather 

Very 
feebly 
negative. 

Com- 
monly 
parallel 

Attacked 
by  H2SO4 

according 

rounded 

synthetic 

sharply 

to  the 

only  with 
difficulty. 

with  an 
angle  of 
110-120° 

sections 
||  oP  rare. 
Elongated 

striations 

\\oP. 
(Fig.  82.) 

the  perfect 
cleavage- 
planes; 

axis  of 
the  cross- 
section. 

(Becke). 

rectangles 

small  axial 

if  the 

angle. 

* 

sections  are 

inclined 

to  oP. 

2.  MONOCLINIC 

act.   PLANE  OF  OPTIC  AXES  GENERALLY  _Lc»Pco;    PERFECT 


1.  a.  Ortho- 

clase. 

K2AI2 

2.50- 

Eminent 

In  grains, 

Very 

A.  P. 

Rather 

In 

Si8016. 

Not 

2-59 
(2-57). 

{tf/** 

or  partly 
columnar: 

common, 
especially 

generally 

-L  ooPoo, 

feebly 
negative. 

sections 
or 

attacked 
by  acids. 
Small 

Cleavage- 
angle 
89°  40'. 

oP.  oo  Poo. 

00  P.  2  POO. 

SPoo.P, 

after  the 
following 
three  laws: 

equally 
inclined 
with  0Pand 

cleavage- 
leaflets  || 
oof  oo  a 

amount  of 

and  partly 

most 

forms  with 

direction 

Na,  Ca,  Fe, 

of  large  or 

commonly 

the  vertical 

of  ex- 

Mg. 

more  rarely 

i.  The 

axis  an 

tinction 

minute 

Carlsbad 

angle  of 

varies 

tabular 

law. 

69°  n'. 

from  the 

crystals. 

Twinning- 

c  =  ^ 

edge  oP  : 

co  Poo.  oo  P. 

plane  oopoo 

a:  a  =  5°. 

ooPoo  = 

oP  2  Poo. 

combined 

True  axial 

a:  a  about 

Poo. 

or  inter- 
penetrated 
in  the  direc- 

angle =  69°. 
(See  Fig.  14.) 
Axial 

5°  18'. 
Sections 

tion  of  the 

dispersion 

parallel  b, 

^-axis.     2. 

=  p  >  v. 

that  is. 

Baveno  law. 

In  sections 

from  the 

Twinmng- 

parallel 

zone  oP: 

plane2Poo, 
especially 

oo  Poo  or 

oo  Poo 

i.  cond.  a 

oo  Poo, 

of  course 
have  a 

columnar 

distorted 

parallel 

examples. 
3.  Rarely 

biaxial  in- 
terference- 

extinc- 
tion. 

the 

figure 

Manebach 

visible. 

law. 

A.  P.  is  rare. 

TABLES  FOR  DETERMINING  MINERALS. 


i65 


Poiariza- 

tlOII- 

coiors. 

Color  and 
power  of 
reiracting 

light. 

Pleo- 
chroism. 

Structure. 

Associa- 
tion. 

Inclosures. 

Decom- 
position. 

Occurrence. 

Remarks. 

Not 

Greenish 

Rather 

Com  pare  with 

With 

Generally 

Rare,  in 

Ottrelite  is 

brilliant, 

black;  in 

power- 

"Inclosures." 

quartz, 

exceed- 

semi- 

triclinic 

similar 
to 
ripidolite. 

sections 
light  to 
grayish 

ful; 
11  oP  lav- 
ender- 

In  large 
tablets  of  a 
black  color 

mica 
(musco- 
vite), 

ingly  rich 
in  in- 

closures 

crystalline 
and 
metamorphic 

(Renard). 
Cleavage  after 
fP\  also  not 

green. 

blue. 

(rather  hard). 

rutile 

of  color- 

schists. 

after  oo  />,  but 

bluish 

Cleavage  ||  oP 

needles, 

less 

intwodirections 

green, 

never  so 

garnet. 

quartz 

cutting  each 

II  * 

very  perfect 

granules^ 

other  at  an 

green- 

as in  the  other 

rutile 

angle  of  about 

ish 

chlorites; 

needles, 

130°,  and  in  a 

blue, 

besides  this, 

and 

third  direction 

yellow- 

however, 

earthy 

at  right  angles 

ish 

always  a 

particles. 

to  one  of  these. 

green. 

cleavage  |]  to 

the  oaxis. 

CRYSTALS. 

CLEAVAGE  [|  oP  AND  cojPoo,  ANGLE  NEARLY  90°. 


Rather 

Rarely 

Orthoclase  in 

With 

As  a  rule 

Mostly 

One  of  the 

The  large 

brilliant, 
not, 

colorless, 
clear  as 

large  crystals 
or  grains  I.O. 

quartz, 
biotite, 

very  poor. 
Hema- 

perfect- 
ly de«- 

most  common 
constituents 

crystals  can 
be  easily  dis- 

however, 

water; 

and  smaller 

musco- 

tite, 

com- 

of  the 

tinguished 

so  bright 
as  in 
quartz. 

generally 
white  or 
opaque 

granules, 
rarely  fila- 
ments, II.  O. 

vite, 
and 
horn- 

biotite 
leaflets, 
fluid  in- 

posed, 
the 
crystals 

granular  and 
porphyritic 
older  eruptive 

from  the 
other  colorless 
optically-biaxial 

In  very 
thin 

from 
decom- 

in eruptive 
rocks;  always 

blende, 
rarely 

closures, 
apatite 

opaque, 
and 

rocks.    Essen- 
tial primary 

minerals  by 
the  twinnings 

sections 

position^ 

in  grains  in 

augite, 

needles, 

non- 

constituent  in 

in  sections 

and  in 
micro- 

gray; 
tinged 

crystalline 
schists. 

plagio- 
clase, 

zircon. 

trans- 
parent; 

granite, 
syenite. 

and  by  the 

lites  the 

red  from 

Penetrations 

elaeo- 

into 

quartzose 

oblique 

polariza- 

ferric 

with  plagio- 

lite. 

kaolin 

porphyry,  and 

extinction 

tion- 

oxide  or 

clase  are 

with 

accessory  in 

parallel  oojPoo.- 

colors  of 

hydrox- 

common, 

forma- 

nearly all 

The  threadlets 

ortho- 

ide. 

generally 

tion  of 

plagioclase 

of  orthoclase 

clase 

||  oo^Poo; 

musco- 

rocks;  more- 

and sanidine, 

are  dull, 
generally 

graphic- 
granite-like, 

vite 
or 

over,  in  the 
crystalline 

so  often 
appearing  in  the 

blue- 
gray, 
as,  e.g., 
nephe- 

with  quartz 
(micro-peg- 
matite).    (See 
Fig.  63,  (?.) 

epidote. 

schists, 
especially  the 
gneisses; 
here  often 

ground-mass 
of  rocks,  have 
often  a  marked 
similarity  to 

hne. 

Zonal  struc- 

glassy, as 

nepheline. 

ture  rare;  also 

sanidine. 

inclo^ures 

zonally 

disposed. 

i66 


DETERMINATION  OF  ROCK-FORMING  MINERALS. 


NAME. 

Chemical 
composi- 
tion and 
reactions. 

Specific 
gravity. 

Cleavage. 

Ordinary 
combinations 
and 
form  of  the 
cross-section. 

Twins. 

Optical 
orientation. 

Character 
and 
strength 
of  double- 
refraction. 

Direction 
of 
extinctioa 

b.  Sani- 

dine. 

As  above; 

Sanidine  in 

Twinning- 

Parallel 

crystals 

minute, 

plane  =  oP. 

ooPoo: 

fur- 
rowed. 

long, 
narrow 

(See  Fig.  28.) 
Cross- 

b  =  * 
a  :  & 

' 

threads,  as 
micro!  ites, 

sections  of 
twins: 

equals  5°. 

or  large 

a.  In  the 

crystals, 

Carlsbad 

never  in 

twins  the 

grains. 

rectangular 

Form  of 

sections  at 

cross- 

right  angles 

sections 

ooPco 

parallel  oP 

divided  into 

and  oo  Poo 

halves 

long  and 
thread-like; 

parallel  to 
the  edges 

parallel 

oP/ooPoo 

coPoo 

and 

distorted 
hexagons 
whose  sides 

00  P/  00  POO. 

b.  In  the 
Baveno 

correspond 
to 

twins  the 
quadratic 

oP.ooP. 

sections 

COPCO. 

at  right 

In  columnar 

angles  oopoo 

types  of 
the  crystals: 

are  divided 
into  halves 

rectangular 
sections  if 

by  the 
diagonals. 

at  right 

angles  to 

tfP.ooPoo, 

octagonal  if 

besides 

these  also 

2Pco   JS 

present. 
(See  Fig.  83.) 

. 

TABLES  FOR  DETERMINING  MINERALS. 


Polariza- 
tion- 
colors. 

Color  and 
power  of 
refracting 

light. 

Pleochro- 
ism. 

Structure. 

Associa- 
tion. 

Inclosures. 

Decompo- 
sition. 

Occurrence. 

Remarks. 

Sanidine 

Sanidine 

Sanidine 

Sanidine 

Almost 

Essential 

and  certain 

is  always 
colorless, 

in  large 
crystals 

like 
ortho- 

generally 
very  rich  in 

always 
fresh, 

primary 
constitu- 

melilites. 
The  isotrope 

clear  as 
-water. 
/3p  = 

I.  O.  and 
minute 
threads  1  1.  0. 

clase; 
besides 
with 

inclosures, 
especially 
vitreous 

rarely 
opaque. 
In 

ent  of  the 
trachytes, 
rhyolites, 

hexagonal 
transverse 
sections  are 

i-^n- 

in  eruptive 
rocks. 
The  large 
crystals 

augite, 
nephe- 
line,  and 
leucite, 

generally 
zonally 
disposed, 

andesites 
and 
trachytes 
a  decom- 

phono- 
lites, 
and  the 
glasses  of 

orthoclase. 
Grains  of 
orthoclase 

are  often 
crumbled  or 

never 

with  mus- 

augite- 
microlites, 

position 
into  opal. 

the  ortho- 

clase 

in  isotrope 
sections 

fused,  and 

covite. 

apatite 

rocks; 

are 

with  an   . 
exceedingly 
beautiful 

needles. 

accessory 
in  nearly 
all  of  the 

easily 
distinguish- 
ed from 

zonal 
structure, 
seen 

younger 
plagio- 
clase 

quartz  by 
the 
condenser, 

particularly 
well. 

rocks, 

as  in 
orthoclase 

1.  p.  p.  1. 

cross- 

Inclosures 

sections 

are  common, 

one  of  the 

arranged 
in  zones. 

optic  axes 
is  visible. 

Orthoclase 

• 

is  distin- 

guished from 

plagioclase 
by  the 

optical 

orientation 

and  absence 

of  the 

polysynthet- 

ic  striation. 

1 68 


DETERMINATION  OF  ROCK-FORMING  MINERALS. 


bb.  PLANE  OF  OPTIC  AXES 


NAME. 

Chemical 
composition 
and 
reactions. 

Specific 
gravity. 

Cleav- 
age. 

O  dinary 
combinations 
and  form  of 
the  cross- 
section. 

Twins. 

Optical 
orientation. 

Character 
and  strength 
of  double- 
refraction. 

Direction 
of 
extinction. 

1.  Mono- 

(-  17-3  4i) 

0 

clinic 

Augite 

Group. 

a.  Ordinary 

RSiO3 

3-34-3-S8- 

Emi- 

Rarely in 

Very 

A.PJooPoo; 

Positive, 

In 

and 
basaltic 

R  =  Mg, 
Ca,  Fe, 

nent 

00  P. 

grains;  in 
crystals: 

common. 
T  win- 

the 
i.  M.  =  cis 

strong. 

sections 
parallel 

Augite. 

and  Fe2O3 

and  A12O3 

00  P.   00^00, 

ooPoo./>, 

ning- 
plane 

wanting 
in  the 

c  :  c  = 

Mixture  of 
CaMg 

and 
sometimes 

^   r> 

also  in 

obtuse 
angle  /3, 

about  39°. 
Varies 

Si^0^+  Ca 

P<x>  .of 

poly- 

b  =  b 

from  39° 

AlaSi06. 
(Tscher- 
mak.) 
/^-rich 

and  -P 

Sections  at 
right  angles 
to  the 
c-axis  are 

synthetic 
twins. 
(See  Figs. 
24  and  25.) 
More 
rarely 

(See 

positive 
axial  angle, 
as  in  the 

to  54°. 
a  :  to  edge 

0/yoojpoo 

about  22°. 
Sections 

augites. 
Not 
attacked  by 

octagonal, 
with 
evident 

penetra- 
tion- 
twins 

rhombic 
augites, 
decreases 

1  b  have 
parallel 
extinc- 

acids. 

prismatic 
cleavage. 
The  longi- 
tudinal 

after: 
twinning- 
plane  a 
face 

with  the 
iron 
present, 
about  60°. 

tion.     In 
sections 
inclined 
to  oojPoo 

sections 
distorted 
hexago- 
nally 
with  the 

—  Pv>\  or 
after: 
twinning- 
plane  a 
face  f>2. 

Sections 
at  right 
angles  to 
the  c-axis 
and 

t:c  de- 
creases to 
0° 
parallel 

c-axis 
parallel  to 

parallel 
<xP<x>  show 

the 

with 

cleavage- 
fissures. 
Also  rect- 
angular 

condenser 
one  optic 
axis  exactly 
in  the 

parallel 

centre  of 

OOjPoo; 

the  field. 

often  a 

rhomb. 

(See 

Fig.  84.) 

TABLES  FOR  DETERMINING  MINERALS. 


:69 


EMINENT  CLEAVAGE  AFTER  o>  p  =  87' 


Polariza- 
tion- 
colors. 

Color  and 

JlllWCf  Of 

1C  light 

Pleo- 
ohrolam. 

Structure. 

Associa- 
tion. 

In- 
closures. 

Decomposi- 
tioii. 

Occurrence. 

Remarks. 

Verv 

In 

Gen- 

In large  crystals 

Princi- 

Vitre- 

Augite 

As  essential 

Easily  dis- 

brilliant, 
especially 
in  the 
light- 
colored, 
yellow  to 
red. 

sections 
green  to 
brown, 
often 
violet  to 
brown  in 
the 

erally 
very 
feeble, 
yet 
augite 
is 
strong- 

I. O.  and  col- 
umns in  micro- 
litesII.O.    The 
first  very  com- 
monly show  a 
zonal  structure, 
a  green  core, 

pally 
with  pla- 
gioclase, 
nephe- 
line, 
leucite, 
with  or 

ous  in- 
closures 
are 
com- 
mon, as 
are 
also 

crystals 
are  com- 
monly de- 
composed 
into  a  pro- 
duct of 
chloritic 

primary 
constituent 
in  many 
younger 
prophyritic 
eruptive 
rocks: 

tinguished 
from  other 
optically- 
biaxial 
minerals 
by  the 
important 

basalts. 

ly  pleo- 

e.g.  with  brown 

without 

gas- 

material, 

diabases, 

oblique 

The 

chroitic 

layers  which  in 

olivine 

pores 

calcite, 

melaphyrs, 

extinction 

same 

as  in 

turn  are  often 

and 

and 

ferric 

augite- 

C  "  C 

crystal 

the 

again  com  posed 

biotite. 

apatite 

hydrate, 

andesites, 

and 

often 

phono- 

of  numberless 

Rarely 

needles. 

epidote, 

and  all 

prismatic 

shows 
several 

lites 
and 

thin  layers.    As 
a  consequence 

with 
ortho- 

Mag- 
netite. 

and  quartz. 
Perfect 

basaltic 
rocks;  also 

c  leafage 
with  angle 

colors. 

then 

ol  this  varying 

clase, 

pseudo- 

common  in 

of  87°; 

(Comp. 

resem- 

constitution 

horn- 

morphs of 

andesites, 

especially 

"struc- 

bles 

of  both  core  and 

blende, 

one  or  more 

trachytes, 

in  trans- 

ture.") 

horn- 

layers, opti- 

and 

of  these 

phonolites. 

verse  sec- 

3p =  1.69. 

blende. 

cal  differences, 

quartz. 

minerals 

Rare  and  in 

tions  ;  more 

Absorp- 

as in  directions 

after  augite 

larger 

difficult 

tion 

of  extinction 

are 

grains  in 

when  gran- 

feeble 

and  polariza- 

common. — 

the  older 

ular;  in  sec- 

c>a>b. 

tion-colors,  are 

Into  opal.  — 

granular 

tions  in- 

a about 

frequent.     (See 

More 

eruptive 

clined  to 

=  b. 

Fig.  45.)    As 

rarely  the 

rocks,  and 

the  oaxis 

with  orthoclase, 

metamor- 

in 

the 

the  successive 

phosis  into 

crystalline 

cleavage- 

layers  in  twins 

hornblende 

schists. 

angle  ap- 

of augite 

(uralitizing) 

proaches 

run  equally  and 
unimpeded 
through  both  in- 
dividuals.  Au- 

wherein 

the  form  of 
augite  re- 
mains with 

that  of 
hornblende. 
Easily  dis- 
tinguished 

?ites  often  show 
the  so-called 

tiornblendic 
cleavage. 

from  epi- 
dote by  the 

"  hour-glass  " 

Finally  the 

color,  direc- 

formation 

rare  meta- 

tion of  ex- 

where sections 

morphosis 

tinction,  re- 

II OO,POO  divide 

into 

lief,  and 

into  four  fields 
of  which  each 

serpentine, 
with  forma- 

polariza- 
tion-colors. 

two  lying  oppo- 
site extinguish 
at  the  same  in- 

tion of 
talc  and 
chlorite. 

If  augite  is 
perfectly 
colorless, 

stant.    (See 

the  polari- 

Figs. 46  and  47.) 
Augite  crystals 

zation- 
colors  are 

are  often  fused, 

very  bril- 

also commonly 
separated  into 

liant  and 
resemble 

large  aggre- 

olivine. 

gates,  the  so- 

called  "  augite- 

eyes,"  or 

needles  radially 

grouped. 

DETERMINATION  OF  ROCK-FORMING  MINERALS. 


Chemical 

Ordinary 

Character 

Direc- 

NAME. 

composition 
and 
reactions. 

Specific 
gravity. 

Cleavage. 

combinations 
and  form  of  the 
cross-section. 

Twins. 

Optical 
orientation. 

and  strength 
of  double- 
refraction. 

tion  of 
extinc- 
tion. 

Z>.  Diallage. 

See  augite. 

3-23- 
3-34- 

oo/>(87°) 
concen- 
trically 
arranged 
after 

Rarely  in 
clearly-defined 
crystals,  mostly 
in  large  tabular 
or  granular 

[|  co  Poo 
poly- 
synthet- 
ic; not 
rarely 

S 

ee  augite. 

oo  fcx>. 

individuals, 

afters/1. 

fibrous  parallel 

to  the  <r-axis. 

c.  Ompha- 
cite. 

d.  Diopside. 

See  augite. 
Rich  in 
A1203. 

More  CaO 
than  MgO, 

3-3- 
3-3- 

See 
augite, 
also  sepa- 
ration 

yet  not  so 
perfect 
as  in 
diallage. 

ditto. 

Only  in  grains 
ditto. 

Rare, 
ditto. 

s 

ditto. 

ee  augite. 
ditto. 

ditto. 

poor  in 

AlaO3. 

Mixture  of 

CaMgSi206 

and 

CaFeSi206. 

* 

(Tscher- 

mak.) 

e.  Sahlite. 

Pale  green 
augite. 

3-2-3-3- 

Separa- 
tion after 

In  grains  and 
long  columns 

ditto. 

ditto. 

ditto. 

ditto. 

poor  in  Fe. 

oP 
together 
with 

with  separation 
at  right  angles 
to  the  longest 

cleavage 

axis,  generally 

after  <»/> 

without 

and 

03  POO, 

terminal  planes. 
Cross-sections 

resemble 

augite. 

TABLES  FOR  DETERMINING  MINERALS. 


171 


Polariza- 
tion- 
colors. 

Color  and 
power  of 
refracting 
light. 

Pleo- 

chruism. 

Structure. 

Associa- 
tion. 

Inclosures. 

Decomposi- 
tion. 

Occurrence. 

Remarks. 

See 
augite. 

Greenish 
brown. 

Very 
feeble. 

Occurs  only 
in  large  ir- 
regular 

With 
plagio- 
clase, 

As  in 
bronzite. 
Inclosures 

Formation 
of  uralite 
common,  in 

Primary 
constituent. 
Common  in 

Often 
resembles 
bronzite. 

grains.     A 
great  simi- 
larity in 
tructure  to 
bronzite  es- 

ordinary 
augite, 
olivine, 
horn- 
blende. 

of  brown 
leaflets 
of  gothite 
parallel 

that 
diallage 
changes  at 
the  ends 
into  dark 

gabbro, 
norite, 
rare  in 
porphyritic, 
eruptive 

Easily  dis- 
tinguished 
from  it  on 
sections  or 
cleavage- 

pecially  as 
regards  in- 
closures, 

Rarely 
with 
quartz. 

otherwise 
poor  in 
inclosures. 

green, 
strongly 
pleo- 

rocks. 
In  serpen- 
tine and 

leaves 

{"'ri 

separating 

chroitic, 

olivine-fels. 

appearance 

into  fibres, 

hornblende 

Rare  in 

of  one  optic 

and  twin- 

fibres. 

crystalline 

axis. 

nings. 

Into 

schists. 

Often  inter- 

viridite; 

penetrated 

nto  serpen- 

with ordi- 

tine  with 

laryaugite, 

formation 

U1           J 

lornblende. 

of  chlorite 

or  mica. 

and  talc. 

Rare  in 

crystals. 

ditto. 

Grass- 

See 

Only 

With 

Rare.  Fluid 

In  eclogites 

See  augite. 

green. 

augite. 

known  in 
resh  grains 
poor  in  in- 

quartz, 
horn- 
blende, 

inclosures 
and  needles 
of  rutile. 

and  amphi- 
bolites. 

They  are 
distinguish 
ed  from  : 

closures; 

garnet, 

aitgite  by 

often  inter- 

zoisite, 

the  paler 

penetrated 

disthene. 

color 

with 

rutile. 

(small 

lornblende. 

amount  of 

Often  en- 

Fe) and  by 

veloped  by 
surround- 

the 
crystalline 

ditto. 

ditto. 

ditto. 

ing  grains. 

With 
olivine, 

Very  rare. 
Vitreous 

As  primary 
constituent 

form; 
diallage  by 
the  lack  of 

chromite. 

inclosures. 

in  olivine- 

the  perfect 

diallage, 

fels  (so- 

separation 

and  the 

called  chro- 

after cofco. 

rhombic 

mium  diop- 

augites. 

side). 

Rarely  sec- 

ondary as 

metamor- 

phic  pro- 

duct of 

garnet. 
(Pyrope.) 

Very 

Pale 

With 

Rarely 

In 

brilliant. 

green  to 
colorless. 

quartz, 
horn- 

changed to 
uralite. 

crystalline 
schists. 

Relief 

blende, 

marked  as 

garnet, 

a  conse- 

scapolite 

quence  o: 

plagio- 

the 

clase, 

powerful 

titanite. 

refraction 

of  light. 

172 


DETERMINATION  OF  ROCK-FORMING  MINERALS. 


NAME. 

Chemical 
composition 
and 

Specific 
gravity. 

Cleavage. 

Ordinary 

combinations 
and  form  of 
the  cross- 

Twins. 

Optical 
orientation. 

Character 
and 
strength  of 
double- 

Direction 
of 
extinction. 

re  ctions. 

sectioii. 

refraction. 

f.  Acorne. 

NaaFea 
Si4012. 

3-53- 
3-55- 

Eminent 

00/>, 

87°: 

In  grains  or 
columns 

<X)P.     <X>PCO, 

com- 
mon. 

S 
A.P.   ||  oojPoo. 
Large  axial 

se  augite. 
Positive. 

C  :  c  =  very 
small 

imperfect 

ca  Pco, 

angle. 

angle 

OOiPoo. 

elongated 

Sections 

=  2-7*. 

by  pre- 

or leaves 

dominance 

||  co  Pco  show 

of  faces 

a  distorted 

oo  Poo. 

axial  picture 

of  a  biaxial 

mineral. 

g.  Woilas- 

CaSiO3 

2.78- 

Parallel 

oo/>  =  87°. 

ditto. 

A.P.   ||  oojPco. 

Positive. 

c  forms 

tonite. 

by  HC1 
perfectly 
decomposed 

2.91. 

00^00,  0P, 

and  /'co. 

Only  in 
prisms, 
irregular, 

Apparent 
axial  angle 
=  about  70°. 

strongly. 

with  oP 
towards 
the  front, 

"with 

elongated, 

(Compare 

32°  12'. 

separation 

fibrous. 

Fig.  ii.) 

a  :  c  =  12°. 

of 

following 

amorphous 
Si02. 

the  /'-ortho- 
axis. 

cc.   PERFECT  CLEAVAGE 

2.  Horn- 
blende 

Group. 
a.  Ordinary 

m  RSiO3 

+ 

3-3- 

Highly 
eminent 

osP.    oojPoo, 

co^oo,  and 

ditto. 

A.P.    ||  ocjPoo. 
The 

Strongly 
negative, 

C  :  c  •=.  about 

and 

*  R2O3. 

00  P. 

oP  '  .  P  or  jPoo 

i.  M.  =  a      yet  some- 

Varies 

basaltic 

R=Ca,  Mg, 

124°  ii'; 

almost 

falls  in  the         what 

from  2-18°. 

Horn- 

Fe. 

imperfect 

always  in 

obtuse 

feebler 

a  :  c  =  75°. 

blende. 

R2  =  Ala, 

OSPCO 

crystals. 

angle  /3, 

than 

Fe2. 
Only  those 

and 
coPoo. 

rarely  in 
grains. 

b  =  b 
(see  Fig.  9). 

augite. 

29°  58'. 

C  :  c  = 

rich  in  Fe 

Transverse 

True  axial 

13-15° 

partially 
attacked  by 
acids. 

sections 
generally 
hexagonal, 

angle  about 
79°,  the  posi- 
tive axial 

in  green 
horn- 
blendes, 

also 

angle  becom- 

and = 

octagonal, 
longitu- 

ing larger 
with  in- 

11-13° 
and  less  in 

dinal 

creased  per- 

brown. 

sections,  as 

centage  of 

in  augite. 

iron. 

(Fig.  86.) 

Parallel  oP 

and  co  Pco 

side  appear- 

ance of  one 

optic  axis  on 

the  circum- 

ference of 

field.     Feeble 

dispersion 

P  <  v. 

b.  Smarag- 

dite. 

See 

uralite. 

TABLES  FOR  DETERMINING  MINERALS. 


1/3 


Color 

Polariza- 
tion- 
colors. 

and 

power  of 
ivt'r.i  cl- 

Ploo- 
chroism. 

Structure. 

Associa- 
tion. 

Inclos- 

urcs. 

Deeom- 
posit.on. 

Occurrence. 

Krmarks. 

ing  light. 

See 

augite. 

Dark 
brown, 

Rather 

strong. 

In  large  crys- 
tals in  the 

With 
elseolite, 

Earthy 
par- 

Not rare  in 
elceolite- 

dark 

cdark 

syenites,  often 

sodalite, 

ticles. 

syenites, 

green. 

brown; 

with  fibrous 

micro- 

phonolites, 

/3  above 

a  brown- 

terminations. 

cline, 

and 

1.7. 

ish 

In  minute  crys- 

and 

trachytes. 

green. 

tals  of  yellow 

biotite. 

Absorp- 

and dark  green 

tion 

color  in  the 

c>  b>  a. 

trachytes  and 

phonolites. 

Very 

bril- 

Color- 
less, 

In  aggregates 
of  fibrous 

With 
calcite, 

Fluid 
inclos- 

As  decom- 
position- 

Resembles 
tremolite,  but 

liant. 

yellow- 
ish 
white. 

individuals  in 
tufts  or  radially 
disposed. 

green 
augite, 
granite. 

ures. 

product  or 
contact-mine- 
ral rare  in 

distinguish- 
able by  the 
prismatic 

Relief 

granular 

angle,  solu- 

marked 

chalks  meta- 

bility in  acids, 

morphosed 
from  eruptive 
rocks.     R;ire 

and  gelatin- 
izing; difficult 
to  distinguish 

in  elaeolite- 

from  zeolites 

syenites  and 

as  scolecite, 

phonolites. 

e-g- 

oo />  =  124°. 


Less 
bril- 

Green 
to 

Generally 
very 

In  large  crys- 
tals or  grains 

With 
ortho- 

Poor  in 
inclos- 

Becomes 
finely 

Primary 
essential  con- 

Easily dis- 
tinguished 

liant 

brown 

strong. 

I.  O.     More 

clase, 

ures. 

fibrous 

stituent.    In 

from: 

than  in 
augite; 
yellow 

/3p  = 
1.62. 

a  = 
yellow- 
green  or 

rarely  in  small 
crystals  and 
microlites  II.  O. 

plagio- 
clase, 
quartz, 

Fluid 
inclos- 
ures, 

and 
bleached 
through 

granular  and 
porphyritic 
eruptive 

augite  by  the 
prismatic 
cleavage- 

to 

honey- 

The  green 

biotite; 

glass, 

decom- 

rocks : 

angle,  slight 

green- 

yellow; 

hornblendes  are 

more 

gas- 

position 

syenite,  dio- 

inclination  of 

ish 

b  = 

often  fibrous; 

rarely 

pores, 

into 

rite  (green 

c  :  c,  and 

brown. 

yellow- 

the  brown  often 

with 

earthy 

epidote, 

hornblende), 

powerful 

brown; 
c  = 

beautifully  de- 
veloped in 

augite 
and 

tides, 

calcite, 
ferric  hy- 

porphyrite, 
trachyte 

pleochroism; 
biotite  on 

black  or 

zones.    The 

olivine. 

apatite 

droxide, 

(brown,  more 

sections  at 

greenish 

brown  horn- 

needles. 

then 

rarely  green, 

right  angles 

brown. 

blende  of  the 

often  sur- 

hornblende). 

to  the  vertical 

Absorp- 
tion 

younger  erup- 
tive rocks  often 

rounded 
by  a 

Accessory  in 
basalts(brown 

axis. 
In  biotite 

c  >  b  >  a. 

shows  a  broad 

wreath 

H.),  rare  and 

the  cleavage 

opaque  margin 

of 

in  olivine-fels 

and  powerful 

(see  Fig.  44),  or 

mag- 

(green H.). 

dichroism  is 

pseudomorphs 

netite; 

Common  in 

wanting  in 

of  augite  and 

always  as 

crystalline 

such  sec- 

magnetite after 

augite. 

schists(green, 

tions  (  ||  oP). 

hornblende 

Meta- 

more rarely 

occur.    The 

mor- 

brown, H.). 

green  horn- 

phoses 

As  essential 

blendes  are 

into 

constituent  in 

often  inter- 

biotite, 

amphibolite, 

penetrated  with 

With 

chlorite. 

hornblendic 

augite. 

ompha- 

schists.certain 

cite, 

gneisses,  ec- 

garnet, 

logite  (so- 

zoisite, 

called  smar- 

rutile. 

agdite). 

174 


DETERMINATION  OF  ROCK-FORMING  MINERALS. 


NAME. 

Chemical 
composition, 
and 
reactions. 

Specific 
gravity. 

Cleavage. 

Ordinary 
combinations 
and  form  of 
the  cross- 
section. 

Twins. 

Optical 
orienta- 
tion. 

Character 
and 
strength 
of  double- 
refraction. 

Direction 
of 
extinction. 

c.  Actinolite 

CaMg3Si4 
012  +  Ca 
Fe3Si4012 

3.026- 
3.160. 

As  above; 
separation 
at  right 

Long  prisms, 
generally 
without 

Rare. 

See  Hor 

nblende. 

C  :  c 
generally 
JS0- 

Al-frce 

angles  to 

terminations 

/>-  poor 

the  c-axis. 

oo  P.  oojPoo. 

(Tschermak). 

RSiO3 

and 

R  =  predomi- 

nating Mg, 

less  Ca,  and  a 

little  Fe. 

d,  Tremo- 
lite. 

3MgSi03 

f    CaSi08. 

MgO  pre- 

2-93-3- 

00   P 

like 
hornblende. 

ooP.  oojPoo 

generally  in 
long  narrow 

Rare. 
Like 
horn- 

See hor 

iblende. 

C:<:  =  15°. 

dominating. 

Separation 

prisms. 

blende. 

Una.tta.cked 

at  right 

by  acids. 

angles  to 

the  c-axis. 

e.  Arfved- 
sonite. 

Na2(Fe)2 
Si4Ol2. 
Insoluble  in 

3-33- 
3-59- 

oo  P  like 
hornblende. 

In  large 
grains. 

Se 

:  hornblen 

de. 

acids. 

i 

f.  Glauco- 
phane 
(Gastal- 
dite). 

Na2(Al)2 
Si4012. 
Contains 
Ca,  Mg,  Fe. 
Nearly 

3-i- 

Like 
hornblende. 
Separation 
at  right 
angles  to 

Elongated 
prisms, 
generally 
without 
terminal 

See  hor 

iblende. 

c:r  =  6i 

-7°  • 

unattacked 

the  c-axis. 

planes. 

by  acids. 

g-.  Uralite 
(Smarag- 
ditempart). 

Like 
ordinary 
green 
hornblendes. 

3-1-3-3- 

Like 
hornblende; 
often, 
however, 

See 
"  Structure;" 
single  fibres 
show  oo  P 

Se 

s  hornblen 

de. 

showing 

=  about  124°. 

in  addition 

Part  in  the 

the  augite- 

form  of 

cleavage 

aiigite  or  in 

quite 

irregular 

perfectly. 

large  grains. 

TABLES  FOR  DETERMINING  MINERALS. 


1/5 


Polariza- 
tion- 
colors. 

Color  and 
power  of 
refracting 

light. 

Pleo- 

chroism. 

Structure. 

Associa- 
tion. 

Inclos- 
ures. 

Decompo- 
sition. 

Occurrence. 

Remarks. 

See 
horn- 

Light to 
dark 

cdark 
green,  a 

Generally 
occurring  in 

With 
quartz, 

Very 
poor. 

Often 
pertect 

Rather 
common  in 

Dis- 
tinguished 

blende. 

green. 

yellowish 
Jreen, 
•ebler 
than  in 

long  narroTV 
needles  or 
grains,  often 
fibrous  at  the 

mica, 
chlorite, 
rulile. 

pseudo- 
morphs  of 
biotite, 
chlorite, 

certain  non- 
feldspathic 
crystalline 
schists. 

from 
ordinary 
green  horn- 
blende by 

the  horn- 

termination. 

and  ferric 

in  talcose, 

chemical 

blendes, 

hydrox- 

mica- 

means; 

generally 

ide  after 

chloritic 

actinolite 

only  in 

actinolite 

schists,  in 

always  oc- 

green 

are 

serpentines. 

curs  in  long 

tints. 

observed. 

columns, 

c  >  b  >  a. 

not  like 

hornblende 

in  short 

crystals. 

Very 

Colorless, 

In  long. 

With    • 

Very 

Into 

As  contact- 

Compare 

brilliant. 

relief 

columns,  the 

calcite; 

poor. 

calcite 

mineral  in 

vvollaston- 

marked. 

termination 

with 

and  talc. 

limestones; 

ite. 

often  in  sheaf- 

olivine, 

as  primary 

like  fibres;  in 

horn- 

constituent 

tufted 

blende, 

also  rarely 

aggregates, 

diallage. 

secondary) 

rarely  in  grains. 

in  crystal- 

line schists 

and 

serpentines. 

See 

Blue- 

Very 

In  irregular, 

With  or- 

Rarely  in 

Dis- 

horn- 

green. 

strong. 

often  fibrous 

thoclase, 

elaeolite 

tinguished 

blende. 

grains  and  long 

micro- 

rocks. 

from  horn- 

columnar 

cline,elae- 

blende  by 

individuals. 

olite, 

chemical 

sodalite. 

composition 

and  color. 

See 
horn- 
blende. 

Indigo-, 
lavender- 
blue. 

Very 
strong. 
a=  white, 

Mostly  in  long 
fibrous  needles, 
often 

With 
quartz, 
horn- 

Rutile 
needles 
and  gas- 

Rare  in 
crystalline 
schists, 

b  =  violet- 

interpenetrated 

blende, 

pores 

eclogitfs, 

blue, 

with  green 

garnet, 

are 

amphibo- 

c  =  dark 

hornblende. 

zoisite, 

com- 

lites, mica 

blue. 

chlorite, 

mon. 

ami 

Absorp- 

ompha- 

chlorite 

tion 

cite, 

schists. 

c  >  b  >  a. 

rutile, 

titanite. 

Generally 
aggregate 
polariza- 
tion, as 
the 

Dark  to 
light 
green. 

Partly 
strong, 
partly 
weak. 

Finely-fibrous 
decomposition- 
product  of 
augite  and 
diallage,  often 

With 
plagio- 
clase, 
olivine, 
diallage, 

In  gabbros 
and 
serpentines; 
in  augitic 
porphyries. 

Compare 
with  dial- 
lage and 
ordinary 
hornblende. 

separate 

of  the  form  of 

augite. 

Anordinary 

horn- 
blende 

augite  and  with 
remnants  of  the 

green 
hornblende 

threads 

augite  or  dial- 

occurring 

have  not 
the  same 

lage  yet  fresh. 
The  fibres  show 

in  eclogue 
was  also 

optical 

the  prismatic 

called 

orienta- 

angle of  horn- 

smaragdite. 

tion. 

blende.     (See 

Fig.  85.) 

76 


DETERMINATION  OF  ROCK-FORMING  MINERALS. 


dd.  CLEAVAGE  ||  0P  AND 


NAME. 

Chemical 
composition 
and 
reactions. 

Specific 
gravity. 

Cleavage. 

Ordinaiy 
combinations 
and  form 
of  the 
cross-section. 

Twins. 

Optical 
orientation. 

Character 
and 
strength 
of  double- 
refraction. 

Direc- 
tion of 
extinc- 
tion. 

Epidote. 

H2Ca4(R2)3 
Si6026 

3-32-3-S- 

Highly 
eminent 

Generally  very 
small  prisms, 

Rare 
micro- 

A. P.  ||  ooPoo 
at  right 

Strongly 
negative. 

a  :  c  = 

2°  10' 

(Ra)=(AI3) 
iFe,). 

Slightly 

\oP, 
and  per- 
fect «>P<* 

elongated  in 
the  direction  of 
ortho-diagonal 

scopi- 
cally. 
Twinning 

angles  to 
the  elonga- 
tion of  the 

C  :  a  = 
27°  47' 
=  C  :  oP. 

attacked  by 

forming 

axis,  the  com- 

plane 

crystal. 

HC1. 

an  angle 

bination  00/>. 

00^00. 

b  =  £,  i.  M. 

of  H5°24'. 

oP  .  f<x>  .  co  Poo 
predominating. 
(See  Fig.  89.) 

(See  Figs. 
26  and  88.) 

=  a  nearly 
coinciding 
with  c. 

The  longitudi- 

nal sections 

Sections  || 

parallel  oofoo 

oo  Poo  show 

are  hexagonal. 

a  biaxial 

The  transverse 

interfer- 

sections at  right 
angles  to  c  and 

ence-figure, 
as  the  2.  M. 

sections  parallel 

is  at  nearly 

oP  :  oo  Poo  are 

right 

long  and  nar- 

1 IgUI. 

angles. 

row,  rectangu- 
lar or 

(See  Fig. 

8?  ) 

hexagonal, 

°/*/ 

with  one  pair 

of  sides  longer  ; 

in  grains. 

ee.  CLEAVAGE  IMPERFECT  oo  p  OR 


Titanite. 

CaSiTiO5  ; 

3-4-36. 

oo  P 

Mostly  crystals: 

Rather 

A.  P.  ||  oojPoo 

Strongly 

o:  c  = 

contains 

*33°  52'» 

oo  P.  oP.  &Poo; 

common; 

i.  M.  =  cat 

positive. 

39°  '7' 

FeO. 

jPw 

\4tP<x>  or  %Pi 

contact- 

nearly right 

tt:a  = 

Decom- 

"3° 3°', 

prominent  with 

or  pene- 

angles to 

21°. 

posed  by 

imperfect. 

oP  .  Poo  .  l^iPoo  . 

tration- 

l^foo; 

HTK)24  ; 
dissolved 
with  forma- 
tion of 
gypsum. 

jPoo,  or  in  acute 
"wedge-sh  aped 
grains.     Such 
are  character- 
istic crystal 
cross-sections. 

twins; 
twinning- 
plane 
=  oP. 
(See  Fig. 
27.) 

very  strong 
dispersion 
of  the  axes. 

P  >    7'. 

(See  Fig. 
13-) 

(See  Fig.  90.) 

TABLES  FOR  DETERMINING  MINERALS.                      177 

%&„ 

Polari- 
zation- 
colors. 

Color  and 
power  of 
refracting 

light. 

Pleo- 
chr^ism. 

Structure. 

Associa- 
tion. 

Inclos- 

ures. 

Decom- 
position. 

Occurrence. 

Remarks. 

Very 
bril- 
liant, 
yellow 

Lemon- 
yellow, 
yello-wish 
green. 

Rather 
powerful 
in  the 
thicker 

Generally 
in  long 
minute 
prisms, 

With 
quartz, 
ortho- 
clase, 

Very 
poor. 
Fluid 
inclos- 

!  Secondary  min- 
eral.    Common 
as  decomposi- 
tion product  of 

Similar  to 
augite,  distin- 
guished from  it 
by  the  even 

to  red. 

Relief 
very 
marked. 
/3  =  1.72- 
1-75- 

prisms. 
a  =  very 
pale 
yellow, 
b=  brown 
to 
yellowish 
green, 

lying  in 
chloritic 
matter, 
or  in 
pseudo- 
morphs, 
more  rarely 
in  grains. 

plagio- 
clase, 
horn- 
blende, 
biotite, 
augite 
with 
chlorite. 

ures. 

the  feldspars, 
hornblende, 
biotite,  more 
rarely  of  augite, 
in  eruptive 
rocks  and  crys- 
talline schists 
bearing  these 

parallel  extinc- 
tion in  sections 
parallel  to  the 
longest 
development 
(=  ^-axis)  and 
slight  inclina- 

c =  green 
to  lemon- 

minerals,  also 
often  as 

tion  of  a.  '.  c. 
The  yellow 

yellow. 
Absorp- 

primary con- 
stituent in  the 

color,  powerful 
refraction  of 

tion 
b  >  c>  a. 

latter. 

light,  and 
brilliant  polari- 

zation-colors 

are  character- 

istic forepidote. 

\ 

poo;  ACUTE  WEDGE-SHAPED  CROSS-SECTIONS. 


Feeble, 

Kray- 

i.e.  like 
original 
color; 
much 

Pale 
yellow, 
reddish 
brown 
to 
colorless. 

Rather 
strong  in 
the  darker 
colored 
varieties, 
tt  =  red- 

Rough sur- 
face of  sec- 
tion is 
characteris- 
tic for 
titanite. 

With 
ortho- 
clase, 
plagio- 
clase, 
horn- 

Very 
poor. 

Rarely 
pseudo- 
morphs 
of 
calcite 
after 

As  primary 
accessory 
constituent  in 
eruptive  rocks. 
Granite 
(rarely), 

Easily 
recognizable 
by  the  almost 
constant  wedge- 
shaped  cross- 
sections, 

weaker 
than 
augite 
and 

dish 
brown. 
C  =  green- 
ish 

Commonly 
associated 
and  inter- 
penetrated 

blende, 
augite, 
biotite, 
chlorite, 

titanite. 

syenite, 
phonolite, 
leucitophyr, 
elaeolite  — 

powerful  refrac- 
tion of  light, 
and  rough 
surface. 

horn- 

yellow. 

with  augite 

quartz, 

syenite, 

blende. 

c>  b>  a. 

and  horn- 

and other 

trachyte,  mica- 

/3p  = 

Weaker 

blende. 

accessory 

andhornblende- 

1.905. 

than  in 

One  of  the 

minerals. 

andesite, 

Relief 

the  horn- 

minerals 

diorite  and 

•uery 

blendes. 

first  formed 

in  crystalline, 

marked. 

in  the 

especially 

eruptive 

hornblendic 

rocks. 

schists. 

Secondary 

as 

decomposition- 

product  of 

ilmenite  and 

titaniferous 

magnetite. 

1 78 


DETERMINATION  OF  ROCK-FORMING  MINERALS. 


NAME. 

Chemical 
composition 
and 
reactions. 

Specific 
gravity. 

Cleavage. 

Ordinary 
combinations 
and  form 
of  the  cross- 
section. 

Twins. 

Optical 
orientation. 

Charactei 
and 
strength 
of  double- 
refraction. 

Direc- 
tion of 
extinc- 
tion. 

Gypsum. 

CaSC>4  + 
2H26. 
Difficultly 
soluble  in 

2.2-2.4. 

Highly 
eminent 
clino- 
diagonal, 

In  granules 
or  elongated 
prismatic 
individu- 

Very rare 
in  micro- 
scopic 
individuals. 

A.P.  ||  oo  Poo 

i.  M.  =  a. 
One  optic 
axis 

Strongly 
negative. 

d:  c  — 
52°  3°'. 
C:  c  — 
37°  3°'- 

acids. 

perfect 

als,  crystals 

nearly 

according    osP.cofoo. 

j_oof  oo. 

to  -  P 

-P. 

One  forms 

83°  with  <:, 

the  other 

22°. 

II.  b.  3.  Minerals  Crystallizing 
a.  LONG  COLUMNAR  CRYSTALS,  COLORLESS  OR  OF  A  BLUE  COLOR, 


Disthene. 

(Cyanite.) 

Al2SiOB. 
Acids  have 
no  action. 

3.48-3.68. 

Highly 
eminent 
II  oo/oo, 

perfect 

00/00, 

Grains,  or 
elongated 
prisms, 

00/00 

predomi- 

Common; 
more  rarely 
on  micro- 
scopic 
individuals. 

A.P.  forms 
with  the 
edge 
oo  PM  :  oP 
an  angle  of 

Rather 
strongly 
negative. 

In 
sections 
parallel 

C0/>00 

and 
parallel 
oP. 

nating, 
oo/oo  with 
an  angle  of 

Twinning 
plane 
either: 

30°;  with 
w/oo:<7/> 
an  angle 

90*7 

(Gleit- 

106°  15', 

I.     00/00 

60°  15',  and 

fldche.') 

rarely  with 
terminal 

repeated; 
z.  At  right 

like  the 
i.  M.  =  a 

planes. 

angles  to 

is  at  right 

Transverse 
sections 

the  oaxis; 
3.  At  right 

angles 

to  oo/oo. 

rectangular 

angles  to 

(See  Fig. 

or 

hexagonal 
if  oo  'P  or 

oo  /"is 

added  to 
the  above 
combina- 

the 2-axis; 
4.  Parallel 
oP,  caused 
by  pressure, 
and  re- 
peated. 

16.)    Large 
axial  angle, 
about  80°. 
Feeble  dis- 
persion of 
the  axes, 
v  <  p. 

tion. 

In  sections 

parallel 

oo/oo  a  bi- 

axial inter- 

ference- 

figure  with 

negative 

middle  line 

is  visible. 

TABLES  FOR  DETERMINING  MINERALS. 


Polari- 
zation' 
colors. 

Color  and 

power  of 
refracting 
light. 

Fleo- 

chroism. 

Structure. 

Association. 

Inclosures. 

Decomposi- 
tion. 

Occurrence. 

Remarks. 

Very 
bril- 
liant. 

Colorlfss, 
secondary, 
often 

In 
minute 
granules, 

Rarely 
with  clastic 
constitu- 

Fluid 
inclos- 
ures. 

As  simple 
rock,  granu- 
lar or 

Irides- 
cent. 

colored  by 
iron 
compounds. 

and  tangled       ents  as 
or  parallel    quartz  gran- 
fibrous            ules  or 

compact. 

aggregates         mica 

of  needles. 

leaflets. 

Rarely  in 

crystals. 

in  the  Triclinic  System. 

OR  GRAINS.     CLEAVAGE  OO^CD  .  oo/'oo  AND  oP. 


Exceed- 

Colorless, 

If  blue 

In  long 

With 

Very 

Rare. 

Rare. 

If 

ingly 
bril- 
liant. 

azure-blue, 
often 
spotted. 
|3p  =  1.72. 
Relief 

rather 
strong- 

pleo- 
chro- 

prisms  or 
irregular 
grains, 
traversed 
by  number 

quartz, 
mica, 
garnet, 
omphacite, 
hornblende. 

poor; 
fluid 
inclos- 
ures. 

Surrounded 
by  a 
marginal 
zone  of  a 
brownish, 

Primary 
accessory 
constituent 
in 
crystalline 

colorless, 
it  is  often 
difficult  to 
distinguish 
from 

marked. 

itic, 
es- 
pecially 
parallel 

00^00. 

blu7. 

less  fissures 
parallel  or 
at  right 
angles  to 
the  chief 
axis,  often 
irregularly 

rarely  with 
orthoclase. 

finely 
fibrous, 
felt-like 
decomposi- 
tion- 
product. 

schists, 
granuhte, 
eclogite, 
and 
especially 
in 
many 

sillimanite, 
with  which 
it 
commonly 
occurs; 
only  possi- 
ble by  the 

a  = 

or  com- 

micaceous 

determina- 

white. 

pletely 

schists. 

tion  of  the 

colored 

position  of 

blue. 

the  axes  of 

Rarely  in 

elasticity. 

ag£regates 
of  thin 

needles  or 

filaments; 

the  needles 

cracked  and 

broken  at 

right  angles 

to  the 

chief  axis. 

ISO  DETERMINATION  OF  ROCK-FORMING  MINERALS. 

ft.  BROAD  TABULAR  CRYSTALS  OR  GRAINS, 


NAME. 

Chemical 
composition 
and 
reactions. 

Specific 
gravity. 

Cleavage. 

Ordinary 
combina- 
tions and 
form  of  the 
cross-section. 

Twins. 

Optical 
orientation. 

Character 
and 
strength  of 
double- 
refraction. 

Direction 
of 
extinction. 

Triclinic 

Feldspars. 

l.Potassium 

Feldspar. 

Microcline. 
(Microper- 
thite, 
so-called 

See  ortho- 
clase. 

2-54- 
2-57 
(2-56). 

Highly 
eminent 
\\oP. 
Eminent 

Very 
similar  to 
orthoclase, 
oo/oo  .  oP. 

Rare. 
Countless 
thin  lamel- 
l<z  of  ortho- 

A. P.  at 
right  angles 
to  oP\ 
its  cross- 

Rather 
strongly 
negative. 
In  leaflets 

C  with  the 
normal  to 

00/00    = 

15°  26'; 

fibrous 
orthoclase.) 

parallel 

00/00, 
00  '/>, 

predomi- 
nating. 

clase  are 
developed 
parallel  to 
oo/oo  and 

section  with 

forms  with 
the  obtuse 

parallel 

oo/oo; 

positive 
double- 

a  cleav- 
age- 
leaflet 
parallel 

at  right 

refrac- 

oP 

angles  to 

oP.  ~C/oo 

tion. 

does  not 

it,  so  that 
in  sections 

5-6°  in 
the  obtuse 

therefore 
extin- 

parallel oP 
a  latticed 

angle  dc. 

guish 
parallel 

inter- 

Cleavage- 

like 

penetration 

leaflets 

ortho- 

of  two 
systems  of 

parallel 

00/00 

clase^ 
but  gives 

striations 

show  one 

an  ex- 

is visible, 

of  the  optic 

tinction 

which  is 

axes  more 

to  the 

exceedingly 
character- 

clearly; the 
axial  plane 

edge 
oP:  oo/oo 

istic  for 

is  some- 

— 

microcline. 

what  in- 

-{-15-16°; 

Besides, 

clined  to 

parallel 

lenticular 

the  plane 

00/00 

amellae  and 

00/00. 

— 

irregular 

+  4-5°- 

lines  of 

polysyn- 

. 

thetic 

twinned 

albite  are 

so  inter- 

penetrated 

that  the 

of  both 

species  of 

plagioclase 

fall  in  one 

plane. 

(See  Figs. 

9I-93-) 

TABLES  FOR  DETERMINING  MINERALS. 


181 


COLORLESS;     CLEAVAGE  PARALLEL  cP  AND 


Polariza- 
tion- 
colors. 

Color  and 

power  of 
refracting 
liKht. 

Pleo- 
chroism. 

Structure. 

Association. 

Inclosures 

Decompo- 
sition. 

Occurrence. 

Remarks. 

Exceed- 
ingly 
bril- 

Colorless. 
Relief 
not  so 

In  rocks  only 
in  grains  ; 
commonly 

a.  With 
orthoclase, 
elaeolite, 

Generally 
very 
poor; 

Fibrous 
decom- 
position 

As 
primary 
essential 

Distinguished 
from: 
orthoclase 

liant. 

marked 

interpene- 

sodalite, 

of  mine- 

with 

constituent 

by  the 

as  in 
ortho- 

trated with 
quartz,  like 

augite, 
and 

rals; 
horn- 

opacity 
as  in 

with 
orthoclase 

oblique 
extinction  on 

clase. 

graphic- 

hornblende. 

blende, 

ortho- 

in : 

0/>,  and  the 

granite. 

biotite, 

clase. 

a.  Elaeolite- 

interpene- 

also  with 

b.  With 

zircon, 

syenite; 

tration 

sodalite  and 

quartz, 

apatite. 

of  twins; 

elaeolite. 

orthoclase, 

b.  In 

the  other 

Compare  the 
twinning 
development. 
An  ortho- 

biotite, 
hornblende, 
muscovite. 

different 
granites, 
especially 
graphic- 

triclinic 
feldspars 
by  the 
latticed 

clase  or 

c.  With 

granite; 

structure 

feldspar 

these  and 

and 

(interpene- 

correspond- 

garnet, 

tration  of 

ing  to 
microcline 

cyanite. 

c.  In 
crystalline 

twins) 
parallel  oP 

was  called 

schists  (as 

and  optical 

microper- 

micro- 

properties. 

thite; 

perthite, 

this  contains 

also 

countless 

called 

exceedingly 
thin  lamellae 

fibrous 
orthoclase), 

of  a  triclinic 

especially 

feldspar 

in 

closely 

granulite 

related  to 

and 

albite,  which 

gneisses. 

' 

can  be 

especially 

well  observed 

in  sections 

parallel 

co/*  oo,  or 

cojpoo,  as 

spindle- 

shaped   cross- 

sections. 

(See  Fig.  93.) 

182 


DETERMINATION  OF  ROCK-FORMING  MINERALS. 


NAME. 

Chemical 
composi- 
tion and 
reactions. 

Specific 
gravity. 

Cleav- 
age. 

Ordinary 
combina- 
tions and 
form  of  the 

cross-section. 

Twins. 

Optical 
orientation. 

Character 
and 
strength  of 
double- 
refraction. 

Direc- 
tion of 
extinc- 
tion. 

2.  Plagio- 

clase, 

Calcium- 

Sodium 

Feld- 

spars: 

a.  Albite 

Ab. 

3.6  l- 

Emi- 

oo/oo .oP. 

Almost  always 

A.  P.  forms 

Rather 

On 

Na2Al2 

2.63 

nent 

oo'P.ao  f". 

twinned. 

with  the 

strongly 

cleav- 

Si.0,e 

with 
traces  of 

(2.62). 

parallel 

oo/oo; 

very  similar 
to 

i.  Albite  law. 
Tw  in  n  ing  -plane 
oo/oo  and  generally 

oaxis  an 
angle  of 
96°  16',  with 

positive. 

age 
pieces: 
parallel 

CaandK, 

1-2%. 

imper- 
fect 

orthoclase. 
(See 

polysynthetic;  there- 
:ore  in  sections  from 

the  normal 
;o  oo/oo  an 

oPthe 
oblique- 

Not at- 

oo /" 

Fig.  94.) 

the  zone  oP\  co/oo 

angle  of 

ness  of 

tacked  by 

and  P,. 

i.  p.  p.  1.  the  single 

16°  17'. 

extinc- 

acids. 
Si02  --= 
68  #. 

Right 
edges 
oP: 

individuals  appear 
as  fine  la  mellce  with 
•varied  polariza- 

i.M. =  C. 
Dispersion 
feeble 

tion  to 
the 
edge 

(and 

00/00  = 

tion-colors.     Only 

p  <  z>; 

oP  : 

oligo- 
clase 

Ab6Anlt 

93°  36'. 

those  sections 
Parallel  oo  /oo  show 

large  axial 
angle. 

+  3°°54~' 

albite). 

no  twinning-stria- 

Cleavage- 

to 

tions.    Two  such 

leaflets 

+  4°  5i' 

polysynthetically- 

parallel 

(+4° 

twinned  albite  indi- 

oo/oo show 

30'); 

viduals  are  often 

quite  com- 

parallel 

again  combined 

plete  dis- 

00/00 

according  to  the 
Carlsbad  orthoclase 

torted  inter- 
fere nce- 

also  = 

twinning-law. 
2.  Pericline  law. 

figure  (ap- 
pearance of 

to  -\-  20° 
(+  19°)- 

Twins  according  to 
the  law  :  axis  of 

the  positive 
middle  line 

rotation  the  ~b-axis^ 

perpendicu- 
lar to 

composition  pla  ne 
the  rhombic  section, 

oc/oo);  yet, 

i.e.,  the  plane  so 
cutting  the  rhom- 
boidal  prism  ao'P. 

the  large 
axial  angle, 

co/*  that  the  plane 
angles  which  these 

position  of 
45°  the 

planes  form  with 
oo/oo  are  equal  to 

hyperbolas 
do  not  lie 

each  other.    The 

in  the  field 

twinning-edge 
hereby  forms  with 

c  inclined 
to  the 

the  edge  <?/>.  oo/oo 
an  angle  of  13-22°. 

sharp  edge 
oP  :  oo/oo. 

Such  twins  are 

(See 

often  again  united 
after  the  Manebach 

Fig.  97.) 

orthoclase  law. 

Also  oP  as  composi- 

tion plane.   By  com- 

bining both  laws  (i 

and  2)  a  latticed 

structure  i.  p.  p.  1. 

is  observed  in  sec- 

tions inclined  to 

oo/oo,  recalling 

that  of  m  icrocline. 

Compare 

Figs.  29  and  30. 

TABLES  FOR  DETERMINING  MINERALS. 


183 


Polariza- 
tion- 
colors. 

Color  and 
power  of 
refracting 
light. 

Pleo- 
chro- 
ism. 

Structure. 

Associa- 
tion. 

Inclosures. 

Decom- 
position. 

Occurrence. 

Remarks. 

For  the 

Colorless, 

In  large 

With 

Very 

Rarely 

Common  in 

The 

most  part 
very 
brilliant. 
Not  so 
powerful 
as  in 
quartz;  in 
very  thin 
sections 
feeble, 
blue- 

clear  as 
water. 
Relief 
feebly 
defined. 
&P=  1-537- 

grains, 
rarely  in 
crystals; 
often  inter- 
penetrated 
with 
orthoclase 
and  quartz. 
Compare 
microcline. 
In 

calcite, 
quartz, 
mica,  and 
ortho- 
clase ; 
chlorite, 
more 
rarely 
with 
horn- 
blende. 

poor; 
fluid  in- 
closures. 

decom- 
posed. 
Fibrous, 
opaque 
decom- 
position. 
See  oligo- 
clase. 

granular 
limestones. 
In 
crystalline 
schists,   in 
many  semi- 
crystalline 
gneisses, 
phyllites, 
sericite- 
schists. 

polysynthetic 
twinning  after 
oo^oo  is  peculiar 
to  all  plagio- 
clase,  and  is 
exceedingly 
characteristic 
for  them. 
The  triclinic 
feldspars  can  be 
distinguished 

green. 

eruptive 

Rare  in 

from  each  other 

rocks  as 

eruptive 

accurately  only 

thin  fibres. 

rocks,  in 

by  chemical 

grains  in 

analysis  or  by 

diorite, 

determination 

in  fibres 
in  many 

of  the  oblique- 
ness of  extinc- 

andesites 

tion  on  oP  and 

and 

oo^oo  on  cleav- 

porphyries . 

age-pieces  from 
grains  or  larger 
crystals.     It 
is  therefore 

impossible  to 

specify  with 

accuracy  the 

minute  plagio- 

clase  threads 

occurring  in  the 

ground-mass  of 

the  eruptive 

rocks;  one  can  at 

best  determine, 

by  measuring 

the  obliqueness 
of  extinction 

in  sections, 

whether  they 

belong  to  a 

plagioclase 

approximating 

albite  or 

anorthite  in 

composition. 

1 84 


DETERMINATION  OF  ROCK-FORMING  MINERALS. 


NAME. 

Chemical 
composition 
and 
reactions. 

Specific 
gravity. 

Cleav- 
age. 

Ordinary 
combinations 
and  form  of 
cross-section. 

Twins. 

Optical 
orientation. 

Character 
and 
strength 
of  double- 
refraction. 

Direction  of 
extinction. 

b.  Oligo- 

clase. 

Si02  = 

62-66. 
per  cent. 

2.62- 
2.65. 
(2.63.) 

Most 
perfect 
II  oP, 

See  albite. 

Always 
polysyn- 
thetic 

Very 
similar  to 
albite.     In 

See 
albite. 

Parallel  oP 
to  the  edge 
oP  :  oo  /oo 

But  little 
K. 
=  AbsAnj 

also 
after 

oo/oo 

twinning 
according 
to  the 

cleavage- 
planes 
parallel 

=  +  i°  10' 
(AbgAnj  = 

-1-  1°  4'); 

to 
Ab3An1. 

like 
albite. 

Albite  laiv; 
also 

00/00 

the  axial 

parallel 

00/00 

oP. 

oo/'oo 

according 
to  the 

points  lie 
still  farther 

to  the  edge 
oP:  oo/oo 

right  = 
93°  28'. 

Pericline 
law. 

beyond  the 
field  than 

=  2-4°, 
(Ab3An1  = 

in  albite. 

4°  36')- 

c  is  inclined 

to  the 

obtuse  edge 
oP:  oo  Poo. 

(See  Fig. 

98.) 

c.  An- 

desine. 

Ab3An1  to 
AbjAnj. 

2.65. 

ditto. 

ditto. 

See  albite. 

Similar  to 
oligoclase, 
yet  with  the 
axial  plane 
more 
strongly 
inclined 
(above  15°) 
to  the 
obtuse 
edge 
oP:  oo/oo. 
Dispersion 
P  <  v. 

ditto. 

Parallel  oP 
to  the  edge 
oP:  oo/~ 
-  i°  57'  to 

—   2°   19'; 

parallel 

00/00 

-*Igto 

TABLES  FOR  DETERMINING  MINERALS. 


I85 


Polari- 
zation- 
colors. 

Color  and 
power  of 
refracting 
light. 

Pleo- 
chroism 

Structure. 

Association. 

Inclosures. 

Decomposi- 
tion. 

Occurri'iuv. 

Remarks. 

See 
albite. 

Colorless 
clear  as 

In  large 
grains  or 

With 
orthoclase, 

Fluid 
inclosures 

Generally 
fresh  in  the 

As  primary 
essential  or 

See  albite. 

water  or 
clouded, 
white, 
grayish 
white. 

crystals 
I.  O.  and  as 
minute, 
elongated, 
and  narrow 

quartz, 
hornblende, 
biotite, 
augite, 
olivine. 

rare,  and 
vitreous 
inclosures 
common 
in  the 

younger 
eruptive 
rocks, 
in  the 
older 

accessory 
constituent 
in  eruptive 
rocks, 
granite, 

threads 
(cross- 
sections  of 

younger 
eruptive 
rocks, 

fibrous  and 
clouded. 
Metamor- 

diorite, 
diabase, 
gabbro, 

thin 
tablets). 

augite-  and 
apatite- 

phosis  into 
epidote 

trachyte, 
andesite, 

Zonal 

microlites. 

called 

also  basalts, 

develop- 

saussurite; 

and  in 

ment  (see 

into  mus- 

crystalline 

Fig.  102) 

covite 

schists, 

and  zonally 

similar  to 

e.g. 

disposed 

orthoclase; 

gneiss. 

inclosures. 

observed 

Almost 

also  in 

always 

nearly  all 

twinned 

plagioclase. 

polysyn- 

thetically. 
Twinning 

and 

concentric 

develop- 

ment 

occurred 

simulta- 

neously as 

in  ortho- 

clase. 

ditto. 

ditto. 

See 

oligoclase. 

With 
sanidine, 

ditto. 

Mostly 
fresh. 

As  primary 
essential 

ditto. 

(Comp. 

orthoclase, 

constituent 

Fig.  102.) 

augite, 

in  tonalite, 

hornblende. 

(quartz- 

biotite, 

diorite),  in 

quartz. 

andesites, 

especially 
dacites  and 

augite- 

andesites, 

porphy- 

rites, 

syenites, 

also  in 

crystalline 

schists. 

186 


DETERMINATION  OF  ROCK-FORMING  MINERALS. 


NAME. 

Chemical 
composition 
and 
reactions. 

Specific 
gravity. 

Cleav- 
age. 

Ordinary 
combina- 
tions and 
form  of  the 
cross-section. 

Twins. 

Optical 
orientation. 

Character 
and 
strength  of 
double- 
refraction. 

Direction 
of 
extinction. 

d.  Labra- 

Ab,  An!  to 

2.68 

See 

Mostly 

The  albite 

In  planes 

Like 

OnoP- 

dorite. 

AbjAn3. 
Si03  = 

55-5  —  49 

-2.70 
(2.69). 

ortho- 
clase; 
often 

in  large 
grains; 
rarely  in 

and 
pericline 
laws 

||  oo/oo  (right) 
a  side  appear- 
ance of  one 

ortho- 
clase. 

-  4°  3°' 
to 
-  6°  54' 

per  cent. 
De- 

change 
of 

crystals;  as 
orthoclase. 

combined 
are 

optic  axis  and 
indication  of 

composable 
byHCl. 

color 
on 

00/00. 

common. 
The 
individuals 

the  lemniscates; 
axial  point 
invisible; 

10');  on 

00/00    = 

-  16°  40' 

twinned 

parallel  oP 

to 

||    CO/00 

side  appearance 

—  21°  I2X 

again 

of  the  other 

(AbiAn, 

twinned 

axis,  the  axial 

=  -  16°). 

after  the 

point  also 

Carlsbad 

invisible. 

law  or 

Dispersion 

according 

p  >  v. 

to  co  Poo 

(See  Fig.  99,  a 

or  oP. 

and  6.) 

See 

structure. 

e.  By- 
townite. 

An.* 

Si02  = 
49-45 
per  cent. 
More 
easily 
soluble  in 
HC1  than  d. 

2.70 

Jke  labrado 

rite. 

Similar  to  labra- 
dorite.     Cleav- 
age-leaflets 
||  oP  and  oo/oo 
show  the  side 
appearance  of 
one  optic  axis, 
the  axial  point 
not  falling 
within  the  field. 
Dispersion 
p  >  71. 
(See  Fig.  100,  a 
and  d.) 

Like 
labrador- 
ite. 

Parallel 
-14.5°  to 

-  20°, 

(AbjAn, 
parallel 

00/00    = 

—  27°  to 
—  32°- 
AbjAn3  = 
-29°  38'. 

TABLES  FOR  DETERMINING  MINERALS. 


I87 


Polari- 
zation- 
colors. 

Color  and 
power  of 
refracting 
light. 

Pleo- 
chro- 
ism. 

Structure. 

Associa- 
tion. 

Inclosures. 

Decom- 
position. 

Occurrence. 

Remarks. 

Gene- 
rally 
very 
bril- 
liant. 

Like 
ortho- 
clase. 

In  grains 
and  large 
crystals  I.O. 
and  microlites 
II.  O. 
Compare 
inclosures 
and  decom- 

With 
diallage, 
hyper- 
sthene, 
olivine, 
also  with 
quartz, 
augite, 

Hornblende, 
olivine, 
diallage, 
magnetite, 
ilmenite. 
Especially 
prominent  are 
the  countless 

Like 

ortho- 
clase. 
Com- 
monly 
into 
epidote 
and 

Primary 
essential 
constituent 
in  norite, 
gabbro, 
dolerite, 
especially 
in  dacite, 

Like 
albite. 

position. 

horn- 

inclosures of 

musco- 

basalts, 

If  labradorite 

blende, 

long  acicular 

vite. 

and 

is  twinned 

biotite. 

opaque  micro- 

diorites. 

according  to 
the  Albite 

lites  disposed 
parallel  to  the 

and  Pericline 

vertical  axis  or 

laws,  a 

also  to  the  edge 

latticed 

oP.aoPoo; 

structure 

also  brownish 

similar  to 

tablets  (ferric 

that  of 

oxide? 

microcline 

brookite?) 

appears  i.p.  1., 

which  lie  with 

yet  the 

their  longer 

twinning 
filaments  in 
labradorite 

direction  per- 
pendicular 
to  the 

are  clearly 

microlites. 

distinguish- 

or countless 

able. 

minute 

colorless  to 

greenish 

granules,  so 

that  the 

labradorite 

appears  opaque 

4 

Likel 

abradori 

te. 

With 
horn- 

Like labrac 
yet  with  no 

lorite, 

Primary 
essential 

blende, 

microlites  and 

constituent 

augite, 

leaflets. 

in  eruptive 

biotite, 

rocks, 

diallage, 
hyper- 

diorite, 
gabbro, 

sthene, 

andesites. 

etc. 

188 


DETERMINATION  OF  ROCK-FORMING  MINERALS. 


NAME. 

Chemical 
composition 
and 
reactions. 

Specific 
gravity. 

Cleavage. 

Ordinary 
combina- 
tions and 
form  of  the 
cross-section. 

Twins. 

Optical 
orientation. 

Character 
and  strength 

of  double- 
refraction. 

Direction 
of 
extinction. 

f.  Anor- 
thite. 

CaAl2Si3 
08.An. 

2.73- 
2-75 

Perfect 

Like  albite. 

i.  M.  =c 
nearly 

Like 
albite. 

Parallel 

Si02  = 
45-43 

(2.75)- 

*«>£>. 

P  :M 

perpen- 
dicular 

-36»to 

per  cent. 
Easily 
soluble  in 

right  = 
94°  10'. 

Dispersion 
p  >  v. 

-37°. 

Parallel 

HC1 

Leaflets 

osfco 

without 

1  */»  and 

=  -37° 

formation 

00/*00 

to  —  43°. 

of  amor- 

show a 

An  = 

phous 

side  ap- 

-36°. 

Si02. 

pearance  of 

one  or  the 

other 

of  the 

optic  axes. 
Axial 

point  on 

margin  of 

the  field. 

(Comp. 

Fig.  101, 

a  and  b.) 

DISTINCTION   BETWEEN 

The  plagioclases  from  b  to  e  inclusive  are,  as  is  well  known,  isomorphous  mixtures  of  the 
terminal  members,  albite  (Ab)  and  anorthite  (An).  As  there  are  all  possible  intermediary  stages 
between  these  two  in  chemical  composition  (oligoclase,  andesine,  labradorite,  bytownite  being  only 
names  for  such  members),  transitions  in  the  physical  properties,  specific  gravity,  and  especially  the 
optical  orientation,  are  also  shown. 

As  has  been  demonstrated  by  M.  Schuster,  one  can  directly  determine  the  proportional  mixture 
of  the  feldspar  to  be  determined,  i.e.,  the  plagioclase  itself,  by  observing  the  directions  of  extinction 
in  cleavage-leaflets  parallel  <?/*and  oopoo. 


C.  Aggre- 

Aggregates  never  show  a  simultaneous  extinction,  i.e.,  total  darkness,  i.  p.  p.  1.  (between 
crossed  nicols  during  a  complete  revolution),  as  the  axes  of  elasticity  of  the  exceedingly  minute 
individuals  forming  the  aggregate  are  irregularly  distributed.  In  a  complete  horizontal  revolution  of 
the  stage,  therefore,  the  separate  individuals  extinguish  in  succession,  and  the  entire  aggregate  does 
not  extinguish  as  a  unit  in  revolving  from  90°  to  90°.  If  the  aggregates  are  radially  fibrous,  an  inter- 
ference-cross is  visible  i.  p.  p.  1. 

In  the  following  pages  something  will  be  given  concerning  the  most  difficultly  determinable 


TABLES  FOR  DETERMINING  MINERALS. 


189 


Polariza- 
tion- 
colors. 

Color  and 
power  of 
refracting 
light. 

Pleochroism. 

Structure. 

Associa- 
tion. 

Inclosures. 

Decomposi- 
tion. 

Occur- 
rence. 

Remarks. 

See 

labrador- 
ite. 

Color- 
less, 
clear  as 
water, 

Like 

labradorite. 

With 
labra- 
dorite, 
augite, 

Like 
oligoclase. 

Generally 
fresh, 
as  in  other 
plagioclase. 

Rather 
rare. 

Primary 
essential 

like 

hyper- 

con- 

labrador- 

sthene, 

stituent 

ite. 

olivine. 

in 

eruptive 

rocks. 

In 

basaltic 

rocks 

and 

augite- 

andesites, 

gabbro, 

norite. 

In  crys- 
talline 

schists, 

amphibo- 

lites, 

gneiss. 

THE  SPECIES  OF  PLAGIOCLASE. 

The  oblique  extinctions  given  have  reference  to  the  usual  setting  up  of  a  plagioclase  (the  oP- 
plane  falling  from  above  forward  and  inclined  from  left  to  right),  and  always  to  the  obtuse  edge 
dP:  oo P<»,  i.e.,  the  plane  °o^cx>  lying  to  the  right.  The  symbol  -f  prefixed  indicates,  on  cleavage- 
leaflets  parallel  oP,  that  the  direction  of  extinction  as  regards  the  right  prismatic  edge  is  inclined 
towards  the  obtuse  oP  :  oo^oo;  on  cleavage- leaflets  parallel  oo/oo,  that  it  is  inclined  towards  the  edge 
oP  :  oo  A»,  the  same  as  the  section  of  the  plane  ,P,<»  with  oo  A».  The  symbol  -  indicates  in  both  cases 
the  opposite  direction. 


gates. 

crypto-crystalline  aggregates.  Their  determination  is  rendered  unusually  difficult  by  the  minuteness 
of  the  separate  individuals;  often  the  chemical  investigation  is  the  only  safe  means  of  determination. 
All  aggregates  here  introduced  are  secondary  minerals,  decomposition-products,  and  often  inclose 
fresh  remnants  of  the  original  mineral.  From  those  minerals  already  studied  aggregates  (crypto-crys- 
talline also)  are  often  formed;  so,  e.g.,  from  talc,  muscovite,  tridymite,  siderite :  these  have  been 
discussed  already  under  the  appropriate  headings. 


190 


DETERMINATION  OF  ROCK-FORMING  MINERALS. 


NAME. 

Chemical 
composition 
and 
reactions. 

Specific  gravity. 

Color  and 
power  of 
refracting  light. 

Optical  properties. 

1.  Serpentine. 

H2Mg3Si208 
-f  aq. 
Completely 
decomposed  by 
HC1. 

2.5-2.7. 

Green,  more 
rarelv  yellow, 
brown,  reddish- 
brown,  black. 
ft  =  1-574- 

Partly 
amorphous, 
partly  showing 
aggregate 
polarization. 
The  variety 
antigorite 
rhombic  (?). 
(See  "structure.") 

Polarization- 
colors  feeble. 
A  ntigorite 
negative 
double-refracting, 
feebly 
pleochroitic. 
Dispersion  clear, 
but  feeble. 

a  =  i.  M.  ±-oP\ 

P>  v. 

perpendicular 
to  the  direction 

of  perfect 

cleavage. 

TABLES  FOR  DETERMINING  MINERALS. 


Structure. 

Association. 

Occurrence. 

Decomposition  • 
product. 

Itt'inark*. 

The  mesh-structure 

With  olivine, 
rhombic  or 

Massive,  as 

For  the  most  part 

Difficult  to 

The  decomposition 
begins  on  the 
walls  of  the 
olivine  fissures; 
generally   yellowish- 
green  threads  shoot 
out  at  right  angles; 
thus  a  sort  of 

monoclinic  augite, 
hornblende, 
garnet,  magnetite, 
chromite,  chlorite, 
magnesite. 

product  of 
olivine-fels;  in 
pseudomorphs 
after  olivine,  in 
olivine-bearing 
eruptive  rocks, 
and  schists. 
As  decomposition- 

a  ugite  free  from 
alumina 
and  hornblende. 

distinguish  from 
the  bastite  and 
chloritic 
decomposition- 
products  of 
augite. 

net  is  formed 

product  of 

embracing  within 
its  meshes  particles 
of  fresh  olivine, 

olivine,  Al-free 
augite,  and 
hornblende. 

which  are 

subject  to  the  further 

decomposition. 

The  interior  of  the 

meshes  generally 
appears  filled  with 

tufted  serpentine 

x 

threads.    The 

mesh-structure  is 

yet  further 

advanced,  in  that 

between  the  single 

fields  earthy 

particles  are 

deposited.     In 

other  serpentines 

the  serpentine 

substance  is 

arranged  in  form  of 

large  often  very 
regular  leaflets 

lying  at  nearly  right 

angles,  showing 

the  optical  behavior 

of  the  so-called 

antigorite:  here  the 

mesh-structure 

is  wanting. 

In  decomposition 

magnetite  is 

separated,  also 

ferric  oxide  and 

hydroxide.    The 

serpentines  are 

often  impregnated 

with  amorphous 

silicic  acid  or 

chalcedony. 

- 

I92 


DETERMINATION  OF  ROCK-FORMING  MINERALS. 


NAME. 

Chemical 
composition  and 
reactions. 

Specific  gravity. 

Color  and  power 
of  refracting 

light. 

Optical  properties. 

2.  Viridite. 
Partly  chloritic, 
partly  serpen- 
tine-like aggre- 
gates, as  : 
a.  Delessite  ; 
b.  Chlorophae- 
ite  ; 
c.  Green  earth 
(Grunerde). 

The  augites  es 
pose  into  dirty  to 
called  by  the  com 
scope  is  impossibl 
viriditic  aggregate 
sometimes  they  a 
ceedingly  fine-gra 

pecially  and  the 
jrownish-green  f 
jrehensive  term 
?  on  account  of  t 
>s  show  aggregat 
re  finely  radial  a 
med  or  more  or 

hornblendes,  als 
ibrous  aggregate 
viridite.     An  ex 
he  minuteness  c 
e  polarization,  £ 
nd  concentric  o 
ess  laminated  a 

3  garnet  and  biotite,  often  decom- 
s,  or,  as  in  green  earth,  granular, 
act  specification  with  trie  micro- 
>f  the  threads  and  grains.     The 
ind  often  a  feeble  pleochroism  ; 
r  tangled  fibrous,  and  again  ex- 
ggregates.     The  three  minerals, 

(Grunerde). 

3.  Bastite. 

Green.     Coinp 
rhombic    pyroxen 
very  like  that  of  « 
especially  the  sepa 

are  with  these 
e  crystals  or  gr 
>erpentine.     Her 
ration-clefts  par 

the  rhombic   py 
ains  into    bastit 
e  also  the  decoi 
illel  oP,  and  pro 

roxenes.     The  decomposition  of 
e  or  a  bastite-like    aggregate  is 
nposition  begins  on  the  fissures, 
presses  into  a  threading  parallel 

4.  Chalcedony. 

SiOa. 

Snail  percentage 
of  H20. 

See  quartz. 

Colorless, 
transparent, 
often  colored 
by  ferric  oxide 
or  hydroxide. 

See  quartz. 

«  =  I-547- 

TABLES  FOR  DETERMINING  MINERALS. 


193 


Structure. 

Association. 

Occurrence. 

Decomposition- 
prouucts. 

Remarks. 

<*,  £,  c  occurring  in  su 
very  commonly  in  ps 
to  the  chemical  comp 
silicate,  and  from  th 
bles  the  chlorites  ;  b 

ch  crypto-crystallint 
eudomorphs  after  ai 
osition,  a  is  a  hydro 
e  high  percentage  c 
and  c  are  iron-ma 

:  aggregates  occur 
jgite.     According 
us  h'eMg  alumina 
i  alumina  resem- 
Ejnesium  silicates, 

For  the  most 
part  from 
monoclinic  augite 
and  hornblende, 

hydrous  and  poor  in 
the  decomposed  basic 

alumina.     Very  wid 
:  eruptive  rocks  and 

ely  distributed  in 
;rystalline  schists. 

garnet,  biotite,  etc. 

to  the  r-axis.     As  a  c< 
of  these  threads  it  i 
bastite  by  studying  i 
under  "  Bastite"  (pag 

jnsequence  of  the  r 
s  often  possible  to  c 
c.  p.  1.     Compare  c 
e  150). 

egular  disposition 
letermine  them  as 
ptical  orientation 

From  the 
rhombic  pyroxenes. 

Chalcedony  is  for 
the  most  part  a 
mixture  of 
amorphous  and 
micro-  or  crypto- 
crystalline  silicic 
acid.    The 
aggregates  are 
either  fine-grained 
or  tangled  fibrous  ; 
also  often  radial. 

Especially  in 
quartz-orthoclase- 
biotite  rocks, 
with  opal  and 
tridymite. 

Secondary 
mineral,  common 
in  the  acidic 
eruptive  rocks, 
especially  rhyolite, 
dacite,  quartz- 
porphyries; 
also  in  other 
decomposed 
eruptive  rocks,  as 
basalt,  andesite, 

A  long  series  of 
minerals, 
especially  the 
feldspars  and 
augite,  yield  on 
decomposition 
chalcedony, 
together  with 
other  products. 

The  primary 
radial  quartz 
sphaerulites  are  to 
be  distinguished 
from  the 
chalcedony  always 
appearing  as 
decomposition- 
product  ;  these 
are  direct 
eliminations  from 

In  the  last  case, 

melaphyr,  and 

the  eruptive 

quartz  individuals 
elongated  according 
to  the  chief  axis  are 

porphyrite,  in 
cavities,  clefts, 
and  irregular 

magma,  and  can 
be  recognized  as 
primary  products 

combined  to  form 

parts  in  the 

from  the  nature 

a  ball,  and  such 
aggregates 

ground-mass. 

of  the  limitations 
(Abgrenzung). 

brilliantly  polarizing 

show  the 

interference-cross 

between 

crossed  nicols. 

194  DETERMINATION  OF  ROCK-FORMING  MINERALS. 


NAME. 

Chemical  composition  and 
reactions. 

Specific  gravity. 

Color  and  power 
of  refracting 
light. 

Optical  properties. 

5.  Zeolites. 

Of  these,  analcime  has 

been  already  sti 

died.    (Cotnpar 

:  Regular  Minerals.) 

a.  Natrolite. 

NaaAl2Si3O10  +  2HaO. 

2.17—2.26. 

Colorless,  clear 
as  water. 
Relief  not 
marked. 

Rhombic. 

b.  Scolecite. 

CaAl3Si3O10-f3HaO. 

2.2-2.39. 

ditto. 

Monoclinic  or  triclinic. 

c.  Stilbite. 

H4CaAlaSi6018  +  3HaO. 

2.1-2.2. 

ditto. 

Monoclinic. 

d.  Desmine. 

CaAl3Si6Oie  +  6H2O. 

2.1-2.2. 

ditto. 

ditto. 

e.  Chabasite. 

RK^O^O. 

All,  a  to  e%  are  easily 
soluble  in  HC1,  with  sepa- 
ration of  gelatinous  silica. 

2.07-2.15. 

ditto. 

Rhombohedral  or 
triclinic.     Rhombohe- 
dral cleavage. 

6.  Carbonates. 

Of  these,  calcite,  dol 
"  Hexagonal  Minerals."    ' 
gates. 

omite,  magnesit 
"hese  occur  also 

;,  siderite,  have 
in  extremely  fine 

been  studied  under  the 
-grained  or  radial  aggre- 

Aragonite. 

CaCO3. 

Easily  soluble  in  HC1  with 
effervescence. 

2.9-3. 

Colorless, 
transparent. 

Rhombic,     Polarization- 
colors  as  in  calcite, 
often  iridescent. 
Cleavage  parallel 
oo  PCD  and  ooP. 
A.P.  U  ooPw;  a  =  c. 

TABLES  FOR  DETERMINING  MINERALS. 


195 


Structure. 

Association. 

Occurrence. 

Decomposition- 
product. 

Remarks. 

Besides  anal 

cime  the  followi 

g  often  occur  a 

»  decomposition- 

products  : 

Almost  always  in  aggre- 
gates of  long  acicular 
crystals,  generally  radially 
disposed  ivitk  brilliant 
polarization-colors. 
A.P.  11  oo/oo  ;  c  =  C. 

Compare 
occurrence. 
With  augite, 
olivine, 
magnetite, 
hornblende, 
biotite, 

Secondary 
minerals, 
especially 
prominent  in 
the  bubble- 
cavities  (see 
Fig.  103)  of 

The  zeolites 
occur 
generally  as 
decomposition- 
products  of  the 
feldspars,  of 
nepheline, 

The  distinctions  are  best 
effected  by  the  micro- 
chemical  examination. 
b  to  d  inclusive  can  be 
accurately  distinguished 
only  by  determining 
the  relation  of  the 

Ditto.   Generally  in  needles 
radially  disposed. 

feldspar,  etc., 
i.e.  their 
decomposition- 
product;  with 

feldspar-, 
nepheline-,  and 
leucite-basalts; 
the  basanites. 

leucite,  and 
hauyn. 

axes  of  elasticity  to  the 
crystallographic  axes 

a  :  c  =  11-12°. 

calcite  and 

tephrites, 

aragonite. 

phonolites.  and 

also  in  trachytic 

and  andesitic 

Tabular  crystals  in  radial 

eruptive  rocks. 

groups,     i.  M.  =  c  =  b. 

As  above.    A.  P.  ||  coPoo. 

b  :  c  =  34°. 

i.  M.  with  a  =  about  5°. 

In  rhombohedra. 

Polarization-colors  like 

feldspar.     Forms  more 

granular  aggregates. 

Siderite  is  very  common  in  spherical,  radial,  and  concentric  aggregates, 
hedral  carbonates  very  commonly  as  decomposition-product  occur  : 


Besides  these  rhombo- 


Partly  in  large  grains  or  in 
radial  fibrous  tufts  of  long 
needles. 

With  calcite 
and  zeolites. 
See  occurrence 

Common  in 
basic  eruptive 
rocks  in  cavities 
and  geodes. 

Decomposition- 
product  of 
calcareous 
silicates. 

Characterized  by 
solubility  with  evolution 
of  CO2,  and  by 
crystalline  form;  easily 
distinguished  from 

calcite  by  the  latter 

property. 

BIBLIOGRAPHY  TO  PART  II. 


The    following    larger    text-books   and    treatises   are   not 
embraced  in  this  bibliography: 

E.  COHEN.     Sammlung  von   Mikrophotographien   zur  Veranschaulichung  der 

mikroskopischen  Structur  von  Mineralien  und  Gesteinen,  aufgenommen 
von  J.  Grimm  in  Offenburg.  Stuttgart,  Schvveizerbart'sche  Verlagshand- 
lung.  1883.  80  Tafeln. 

FISCHER.  Kritische  mikroskop.-mineralogische  Studien.  3  Hfte.  Freiburg  i. 
Br.  1869-1873. 

F.  FOUQUE  et  A.  MICHEL  LEVY.    Mineralogie  micrographique  roches  eruptives 

frangaises.   Paris,  1879.  a.  Atlas  LV  PI. 

H.  ROSENBUSCH.  Mikroskopische  Physiographic  der  petrographisch  wichtigen 
Mineralien.  Stuttgart,  Schweizerbart'sche  Verlagshandlung.  1873.  Mit 
10  Tafeln. 

—  Mikroskopische  Physiographic  der  massigen  Gesteine.   Stuttgart,  Schvveizer- 

bart'sche Verlagshandlung.    1877. 

F.  ZIRKEL.  Die  mikroskopische  Beschaffenheit  der  Mineralien  und  Gesteine. 
Leipzig,  W.  Engelmann.  1873. 

—  Microscopical  Petrography.  Washington,  1876.  w.  XII  PI. 

Acmite  and  Aegirine. 

TSCHERMAK.  Tschermak's  Mineral.   Mitth.   1871.  33. 

BECKE.    Tschermak's  Mineral   u.  petr.  Mitth.   N.  F.   I.   1878.  554. 

KOCH.    N.  Jahrbuch  f.  Min.  u.  Geol.    1881.   I.  Beil.-Bd.   156. 

TORNEBOHM.    Forh.  geol.  Foren.  i  Stockholm.    1883.  VI.    383  and  542.   Comp. 

Ref.  N.  Jahrb.  f.  Min.  u.  Geol.   1883.   II.  370. 
MttGGE.    N.  Jahrb.  f.  Min.  u.  Geol.    1883.   II.  189. 
MANN,  N.  Jahrb.  f.  Min.  u.  Geol.   1884.   II.   172. 


198       DETERMINATION  OF  ROCK-FORMING  MINERALS. 

Actinolite  (Smaragdite,  Karinthine). 

TSCHERMAK.    Tschermak's  Min.  Mitth.   1871,  37  and  44. 
v.  DRASCHE.    Tsch.  Min.  Mitth.   1871.  85. 
RIESS.    Tsch.  Min.  u.  petr.  Mitth.  N.  F.   1878.  I.   185,  192. 
CH.  WHITMAN  CROSS.    Tsch.  Min.  u.  petr.  Mitth.   1881.  III.  386. 
BECKE.     Tsch.  Min.  u.  petr.  Mitth.   1882.   IV.   234,  360. 
-  Tsch.  Min.  u.  petr.  Mitth.   1882.  V.   157. 

Albite. 

LOSSEN.  Zeitschr.  d,  deutsch.  geol.  Ges.   1.867.  XIX.   509  and  1879.  XXXI.  441. 
SCHUSTER.    Tsch.  Min.  und  petr.  Mitth.   N.  F.   1881.   III.   153. 
BOHM.    Tsch.  Min.  und  petr.  Mitth.   N.  F.  1883.  V.  202. 

Almandine  (ordinary  Garnet). 

DRASCHE.    Tsch.  Min.  Mitth.   1872.   2.  85. 

WICHMANN.    Pogg.  Ann.  f.  Phys.  u.  Chem.   1876.  CLVII.  282. 

DATHE.    Zeitschr.  d.  deutsch.  geol.  Ges.   1877.  XXIX.  274. 

RIESS.   Tsch.  Min.  u.  petr.  Mitth.   1878.   I.   186. 

SZABO.    N.  Jahrb.  f.  Min.  u.  Geol.   1880.   I.   Beil.-Bd.   302. 

SCHRAUF.    Groth's  Zeitschr.  f.  Kryst.   1882.  323. 

RENARD.    Bull,  du  Musee  royal  d'hist.  nat.  de  Belgique.   1882.  I. 

v.  LASAULX.    Sitzungsber.  d.  niederrhein.  Ges.  in  Bonn.   1883. 

Analcime. 

TSCHERMAK.    Sitzungsber.  Wien.  Akad.  d.  Wiss.  1866.  LI II.  260. 

Andalusite. 

JEREMEJEFF.    N.  Jahrb.  f.  Min.  u.  Geol.   1866.   724. 

ZIRKEL.    Zeitschr.  d.  deutsch.  geol.  Ges.   1867.  XIX.  68.   180. 

ROSENBUSCH.    Die  Steiger  Schiefer.  Strassburg,  1877. 

POHLIG.    Zeitschr.  d.  deutsch.  geol.  Ges.   1877.  XXIX.   560,  and  Tsch.  Min.  u. 

pe*r.  Mitth.   1881.  III.  344. 

TELLER  u.  JOHN.    Jahrb.  d.  kk.  geol.  R.-Anst.  Wien,  1882.  XXXII.  589. 
MULLER.    N.  Jahrb.  f.  Min.  u.  Geol.   1882.  II.  205. 

Andesine. 

v.  RATH.    Zeitschr.  d.  deutsch.  geol.  Ges.   1864    XVI.  294. 
SCHUSTER.    Tsch.  Min.  u.  petr.  Mitth.   1881.   III.   173. 
BECKE.    Tsch.  Min.  u.  petr.  Mitth.   1882.  V.   149,  160. 


BIBLIOGRAPHY    TO  PART  II.  199 

Anomite. 

TSCHERMAK.    Gr.  Zeitschr.  f.  Kryst.  1878.  31. 

BECKE.    Tsch.  Min.  u.  petr.  Mitth.   1882.   IV.  331.  V.  151. 

Anorthite. 

BECKE.    Tsch.  Min.  u.  petr.  Mitth.   1882.   IV.  246. 
SCHUSTER.    Tsch.  Min.  u.  petr.  Mitth.   1881.   III.  208. 

Anthophyllite. 

TSCHERMAK.    Tsch.  Min.  Mitth.  1871.  37. 

CH.  WHITMAN  CROSS.    Tsch.  Min.  u.  petr.  Mitth.   1881.   III.  388. 

BECKE.    Tsch.  Min.  u.  petr.  Mitth.   N.  F.   1882.   IV.  331.  450. 

SJOGREN  (on  Gedrit).    Comp.  Ref.  N.  Jahrb.  f.  Min.  u.  Geol.   1883.   II.  366. 

Apatite. 

ROSENBUSCH.    Nephelinit  v.  Katzenbuckel.   Freiburg  i.  Br.  1869. 
ZIRKEL.    Basaltgesteine.   Bonn,  1870.   72. 

—  N.  Jahrb.  f.  Min.  u.  Geol.   1870.  806,  821. 
HAGGE.    Ueber  Gabbro.   In.-Diss.  Kiel,  1871.  58. 
KREUTZ.    Tsch.  Min.  u.  petr.  Mitth.  N.  F.   1884.  VI.   149. 

Arfvedsonite. 

KOENIG.    Gr.  Zeitschr.  f.  Krystall.   1877.  423. 

Augite  (ordinary  and  basaltic). 

WEDDING.    Zeitschr.  d.  deutsch.  geol.  Ges.   1858.  380. 
BUTSCHLY.    N.  Jahrb.  f.  Min.  u.  Geol.    1867.   700. 
TSCHERMAK.    Sitzungsber.  d.  Wien.  Akad.  d.  Wiss.  1869.  LIX. 

—  Tsch.  Min.  Mitth.    1871.   28. 
ROSENBUSCH.    Neph.  v.  Katzenbuckel.   1869. 
ZIRKEL.    Basaltgesteine.   1870.   8. 

VRBA.    Zeitschr.  "  Lotos"  Prag.  Jahrg.   1870. 

DATHE.    Zeitschr.  d.  deutsch.  geol.  Ges.   1874.  XXVI.   i. 

LAGORIO.    Andesite  d.  Kaukasus.  Dorpat,  1878.   Ref.  N.  Jahrb.  f.  Min.  u.  Geol. 

1880.   I.   209. 

v.  WERVEKE.    N.  Jahrb.  f.  Min.  u.  Geol.   1879.  482.  822. 
BECKE.    Tsch.  Min.  u.  petr.  Mitth.   1882.   IV.  365. 
KREUTZ.    Tsch.  Min.  u.  petr.  Mitth.   1884.  VI.   141. 


200      DETERMINATION  OF  ROCK-FORMING  MINERALS. 

Bastite. 

TSCHERMAK.    Tsch.  Min.  Mitth.   1871,  20. 
HAGGE.    Ueber  Gabbro.  Kiel,  1871.  27. 
STRENG.    N.  Jahrb.  f.  Min.  u.  Geol.   1872.  261. 
DRASCHE.    Tsch.  Min.  Mitth.   1873.  5. 

Bronzite  (comp.  with  Serpentine). 

TSCHERMAK.    Sitzungsber.  d.  Wien.  Akad.  d.  Wiss.   1869.  LIX.  I.  I. 

—  Tsch.  Min.  Mitth.   1871.   17. 

STRENG.    N.  Jahrb.  f.  Min.  u.  Geol.   1872.  273. 

SCHRAUF.    Gr.  Zeitschr.  f.  Kryst.   1882.  321. 

BOOKING.    Gr.  Zeitschr.  f.  Kryst.   1883    VII.  502. 

BECKE.    Tsch.  Min.  u.  petr.  Mitth.   1883.  V.  527. 

ROSENBUSCH.    N.  Jahrb.  f.  Min.  u.  Geol.   1884.  I.   197. 

Bytownitel 

SCHUSTER.    Tsch.  Min.  u.  petr.  Mitth.   1881.   III.  202. 

BECKE.    Tsch.  Min.  u.  petr.  Mitth.  1882.  V.   168. 

RENARD.    Bull.  d.  Musee  roy.  d'hist.  nat.  belgique.   1884.  III.  10. 

Calcite. 

OSCHATZ.    Zeitschr.  d.  deutsch.  geol    Ges.   1855.  VII.   5. 

STELZNER.    Ueber  Gesteine  v.  Altai.   Leipzig,  1871.  Aus  Cotta:  D.  Altai,  p.  57. 
INOSTRANZEFF.    Tsch.  Min.  Mitth.  1872.   I.  45. 
ROSENBUSCH.    N.  Jahrb.  f.  Min.  u.  Geol.  1872.  64. 
LEMBERG.    Zeitschr.  d.  deutsch.  geol.  Ges.  1872.  226.   1876.  519. 
LAGORIO.     Mikrosk.  An.  ostbaltischer  Gebirgsarten.  Dorpat,   1876. 
O.  MEYER.    Zeitschr.  d.  deutsch.  geol.  Ges.   1879.  XXXI.  445. 
RENARD.    Bull.  Acad.  royal  des  Sciences  belg.    1879.    XLVII.    Nr.  5.    Comp. 
Ref.  N.  Jahrb.  f.  Min.  u.  Geol.   1880.   II.   146. 

Cancrinite. 

A.  KOCH.    N.  Jahrb.  f.  Min.  u.  Geol.   1881.   I.   Beil.-Bd.   144. 
TORNEBOHM.     Geol.  Foren.  i  Stockholm  Forh.    1883.  VI.   383.  Comp.  Ref.    N. 
Jahrb.  f.  Min.  u.  Geol.   1883.  II.  370.  542. 

Chalcedony. 

REUSCH.    Pogg   Ann.  f.  Ph.  u.  Chem.   1864.  CXXIII.  94 

BEHRENS.    Sitzungsber.  d.  Wien.  Akad.  d.  Wiss.  1871.   LXIV.  Dec.  i. 


BIBLIOGRAPHY  TO  PART  II.  2OI 

&D 
$1 


Chiastolite.  \  „. 


ZIRKEL.    Zeitschr.  d.  deutsch.  geol.  Ges.   1867.  68. 

POHLIG.    Zeitschr.  d.  deutsch.  geol.  Ges.   1877.  XXIX.   545.   563. 

—  Tsch.  Min.  u.  petr.  Mitth.   1881.   III.  348. 

CH.  WHITMAN  CROSS.    Tsch.  Min.  u.  petr.  Mitth.   1881.   III.  381. 
MULLER.    N.  Jahrb.  f.  Min.  u.  Geol.   1882.  II.  205. 

Chloritoid  (Sismondine). 

TSCHERMAK  u.  Sipocz.     Gr.  Zeitschr.  f.  Kryst.   1879.   5°6  and  509. 
v.  FOULLON.    Jahrb.  d.  kk.  geol.  R.-Anst.     Wien,  1883.  XXXIII.  207. 
BARROIS.    Ann.  de  la  Soc.  geol.  du  Nord.    Lille,  1883.   XI.   18.  Comp.  Ref.  N. 
Jahrb.  f.  Min.  u.  Geol.   1884.   II.  68. 

Chromite. 

DATHE.    J.  Jahrb.  f.  Min.  u.  Geol.  1876.  247. 
THOULET.    Bull.  Soc.  miner.  Paris,  1879.  34. 

Oordierite. 

WICHMANN.    Zeitschr.  d.  deutsch.  geol.  Ges.   1874.  XXVI.  675. 
v.  LASAULX.     N.  Jahrb.  f.  Min.  u.  Geol.   1872.  831. 

—  Gr.  Zeitschr.  f.  Kryst.   1883.  VIII.  76. 

SZABO.     N.  Jahrb.  f.  Min.  u.  Geol.   1880.   I.  Beil.-Bd.  308. 
HUSSAK.     Sitzungsber.  d.  Wien.  Akad.  d.  Wiss.   1883.  April. 
CALDERON  y  ARANA.    Bal.  d.l.  Comis.  d.  Mapa.  geolog.  Madrid,  1882. 

Corundum. 

ZIRKEL.    N.  Jahrb.  f.  Min.  u.  Geol.    1870.  822. 

TELLER  u.  JOHN.    Jahrb.  d.  kk.  geol.  R.-Anst.  Wien,  1882.  XXXII.  589. 

WICHMANN.    Verhandl.  d.  kk.  geol.  R.-Anst.  Wien,  1884.   150. 

Couseranite  (Dipyr). 

ZIRKEL.    Zeitschr.  d.  deutsch.  geol.  Ges.   1867.  XIX.  202. 
GOLDSCHMIDT.    N.  Jahr.  f.  Min.  u.  Geol.   iSSr.   I.  Beil.-Bd.  225. 

Diallage. 

G.  ROSE.    Zeitschr.  d.  deutsch.  geol.  Ges.   1867.   280,  294. 

TSCHERMAK.    Tsch.  Min.  Mitth.   1871.    25,  and  Sitzungsber.  d.  Wien.  Akad.  d. 
Wiss.   1869.   LIX.   i.   i. 


202       DETERMINATION  OF  ROCK-FORMING  MINERALS. 

v.  DRASCHE.    Tsch.  Min.  Mitth.   1871.   i. 

HAGGE.    N.  Jahrb.  f.  Min.  u.  Geol.   1871.  946. 

STRENG.    N.  Jahrb.  f.  Min.  u.  Geol.   1872.  377.  379. 

v.  RATH.    Verb.  d.  niederrhein.  Ges.  f.  Nat.  u.  Heilkde.   Bonn.  8.  Mz.    1875. 

DATHE.    Zeitschr.  d.  deutsch.  geol.  Ges.   1877.  XXIX.  274. 

SCHRAUF.    Gr.  Zeitschr.  f.  Kryst.   1882.  323. 

v.  WERVEKE.    N.  Jahrb.  f.  Min.  u.  Geol.   1883.   II.  97. 

KLOOS.    N.  Jahrb.  f.  Min.  u.  Geol.   1884.   III.  Beil.-Bd.  19. 


Diopside  (Omphacite  and  Sahlite). 

TSCHERMAK.    Tsch.  Min.  Mitth.   1871.  21. 

v.  DRASCHE.    Tsch.  Min.  Mitth.   1871.  58. 

v.  KALKOWSKY.    Tsch.  Min.  Mitth.  1875.  II. 

DATHE.     N.  Jahrb.  f.  Min.  u.  Geol.   1876.   225,  337. 

RIESS.    Tsch.  Min.  u.  petr.  Mitth.   1878.  I.   168. 

BECKER.     Zeitschr.  d.  deutsch.  Geol.  Ges.   1881.  XXXIII.  31. 

BECKE.    Tsch.  Min.  u.  petr.  Mitth.  1882    IV.  297. 

SCHRAUF.    Gr.  Zeitschr.  f.  Kryst.  1882.  321. 


Disthene  (Cyanite). 

V.  KOBELL     Pogg.  Ann.  f.  Phys.  u.  Chem.   1869.   CXXXVI.   156. 
V.  LASAULX.     N.  Jahrb.  f.  Min.  u.  Geol.   1872.  835. 
RIESS.     Tsch.  Min.  u.  petr.  Mitth.   1878.   I.   165,  195. 
BECKE.    Tsch.  Min.  u.  petr.  Mitth.  1882.  IV.  225,  231. 


Dolomite. 

INOSTRANZEFF.    Tsch.  Min.  Mitth.   1872.  48. 
LEMBERG.    Zeitschr.  d.  deutsch.  geol.  Ges.   1876.  519. 
O.  MEYER.    Zeitschr.  d.  deutsch.  geol.  Ges.   1879.  445- 

RENARD.     Bull.  Acad.  royal  Belg.  XLVII.  5.  Mai  1879.  Comp.  Ref.  N.  Jahrb. 
f.  Min.  u.  Geol.   1880.  II.   146. 


Elseolite. 

SCHEERER.     Pogg.  Ann.  f.  Phys.  u.  Chem.   1863.  CXIX.  145. 
ZIRKEL.     N.  Jahrb.  f.  Min.  u.  Geol.    1870.   810. 
v.  WERVEKE.     N.  Jahrb.  f.  Min.  u.  Geol.   1880.   II.   141. 
KOCH.     N.  Jahrb.  f.  Min.  u.  Geol.   1880.   I.  Beil.-Bd.   140. 


BIBLIOGRAPHY  TO  PART  II.  203 

Enstatite. 

TSCHERMAK.     Tsch.  Min.  Mitth.   1871.   17. 

STRENG.    N.  Jahrb.  f.  Min.  u.  Geol.   1872.  273. 

TRIPPKE.    N.  Jahrb.  f.  Min.  u.  Geol.  1878.  673. 

TELLER  u.  JOHN.    Jahrb.  d.  kk.  geol.  R.-Anst.  Wien,  1882.  XXXII.  589. 

Epidote. 

ZIRKEL.    Zeitschr.  d.  deutsch.  geol.  Ges.   1869.  XIX.   121. 
v.  LASAULX.     N.  Jahrb.  f.  Min.  u.  Geol.   1872.  837. 
BECKE.    Tsch.  Min.  u.  petr.  Mitth.  1879.  II.  25,  34. 
-  Tsch.  Min.  u.  petr.  Mitth.   1882.   IV.  264. 
v.  KALKOWSKY.    Tsch.  Min.  u.  petr.  Mitth.   1876.   II.  87. 
REUSCH.    N.  Jahrb.  f.  Min.  u.  Geol.   1883.  II.   179. 
TORNEBOHM.    Geol.  Foren.  i  Stockholm  Forh.  VI    185.  Comp.  Ref.  N.  Jahrb. 

f.  Min.  u.  Geol.   1883.   I.   245. 

BACHINGER.    Tsch.  Min.  u.  petr.  Mitth.   1884.  VI.  44. 
KticH.    Tsch.  Min.  u.  petr.  Mitth.   1884.  VI.   119. 

Fluorite. 
LASPEYRES.    Zeitschr.  d.  deutsch.  geol.  Ges.   1864.  XVI.  449. 

Glaucophane. 

HAUSMANN.    Gottinger  gel.  Anz.   1845.  195. 

BODEVVIG.     Pogg.  Ann.  f.  Phys.  u.  Chem.   1876.  CXLVIII.  224. 

LUEDECKE.    Zeitschr.  d.  deutsch.  geol.  Ges.   1876    XXVIII.  248. 

BECKE.    Tsch.  Min.  u.  petr.  Mitth.   1879.   U.  49,  71. 

WILLIAMS.    N.  Jahrb.  f.  Min.  u.  Geol.   1882.   II.  201. 

STELZNER.     N.  Jahrb.  f.  Min.  u.  Geol.   1883.   I.  208. 

BARROIS.    Ann.  Soc.  geol.  du  Nord.   Lille.  1883.  XI.   18.  Comp.  Ref.  N.  Jahrb. 

f.  Min.  u.  Geol.   1884.   II.  68. 
v.  LASAULX.    Sitzungsber.  d.  niederrhein.    Ges.  F.  Nat.  u.  Heilkunde.    Bonn, 

1884.  3.  XII. 

Graphite. 

ZIRKEL.    Zeitschr.  d.  deutsch.  geol.  Ges.  1867.  68. 
—  Pogg.  Ann.  f.  Phys.  u.  Chem.   CXLIV.   1871.  319. 

RENARD.     Bull,  du  Musee  royal  d'hist.  nat.  Bruxelles,  1882.  I.  47.  Comp.  Ref. 
N.  Jahrb.  f.  Min.  u.  Geol.   1883.   II.  68. 


2O4       DETERMINATION  OF  ROCK-FORMING  MINERALS. 

Gypsum  (and  Anhydrite). 
HAMMERSCHMIDT.    Tsch.  Min.  u.  petr.  Mitth.   1883.  V.  245. 


Hauyn  (comp.  Nosean). 

ZIRKEL.     Basaltgesteine.    1870.  79. 
-  N.  Jahrb.  f.  Min.  u.  Geol.   1870.  818. 

VOGELSANG.   Mededeel.  d.  k.  Akad.  v.  Wetenschapp.  Amsterdam,  1872  (2). 
SAUER.    Zeitschr.  f.  d.  gesammt.  Naturwiss.   Halle,  1876.  XIV. 
DOELTER.    Tsch.  Min.  u.  petr.  Mitth.   1882.  IV.  461. 

Hematite. 

G.  ROSE.    Zeitschr.  d.  deutsch.  geol.  Ges.  1859.  XI.  298,  306. 
KOSMANN.    Zeitschr.  d   deutsch.  geol.  Ges.   1864.  XVI.  665. 
ZIRKEL.    Basaltgesteine.  1870.  71. 

Hercynite. 

v.  KALKOWSKY.    Zeitschr.  d.  deutsch.  geol.  Ges.  1881.  XXXIII.  533. 

Hornblende  (ordinary  and  basaltic). 

ZIRKEL.     Zeitschr.  d.  deutsch.  geol.  Ges.   1867.   99.   119. 

—  Zeitschr.  d.  deutsch.  geof.  Ges.   1871.  43. 
-  Basaltgesteine.   1870.  74. 

TSCHERMAK.     Sitzungsber.  d.  Wien.  Akad.  d.  Wiss.   1869.   LIX.  i.   I. 

—  Tsch.  Min.  Mitth.   1871.  38. 

RIESS.    Tsch.  Min.  u.  petr.  Mitth.   1878.   165. 

SOMMERLAD.     N.  Jahrb.  f.  Min.  u.  Geol.   1882.   II.   139. 

BECKER.    N.  Jahrb.  f.  Min.  u.  Geol.  1883.   II.   i. 

STRENG.    XXII.  Bericht  d.  oberhess.  Ges.  f.  Natur-  u.  Heilkunde.  Giessen,  1883. 

KLOOS.    N.  Jahrh.  f.  Min.  u.  Geol.   1884.  III.  Beil.-Bd.  24. 

Hypersthene. 

KOSMANN.    Sitzungsber.  d.  niederrhein  Ges.  f.  Natur-  u.  Heilkunde.     Bonn, 
3.  Febr.  1869. 

—  N.  Jahrb.  f.  Min.  u.  Geol.   1869.   374  and  1871.   501. 
HAGGE.    N.  Jahrb.  f.  Min.  u.  Geol.  1871.  946. 
TSCHERMAK.    Tsch.  Min.  Mitth.  1871.  17. 


BIBLIOGRAPHY   TO  PART  II.  2O5 

NIEDZWIEDZKI.    Tsch.  Min.  Mitth.   1872.  253. 

BECKE.    Tsch.  Min.  u.  petr.  Mitth.   1878.  I.  244. 

BECKE.     Tsch.  Min.  u.  petr.  Mitth.   1883.  V.  527. 

FOUQUE.     Santorin.   Paris,  1879. 

BLAAS.     Tsch.  Min.  u   petr.  Mitth.   1881.   III.  479. 

TELLER  u.  JOHN.     Jahrb.  d.  kk.  geol.  R.-Anst.  Wien.   1882.  XXXII.   589. 

ROSENBUSCH.     Gesteine    v.    Ekersund.      N.    Magaz.    f.     Naturvidenskaberne. 

XXVII.  4.   Heft. 
HAGUE  u.  IDDINGS.     Amer.   Journ.   of  Science.    1883.    XXVI.    222.    Ref.  N. 

Jahrb.  f.  Min.  u.  Geol.   1884.   I.  225. 
CH.  WHITMAN  CROSS.     The  same.  XXV.   1883.  139.  Ref.  N.  Jahrb.  f.  Min.  u. 

Geol.   1884.   I.  228. 
KRENNER.    Gr.  Zeitschr.  f.  Kryst.    1884.  IX.  255. 

Ilmenite. 

LASPEYRES.     N.  Jahrb.  f.  Min.  u.  Geol.   1869.  513. 

ZIRKEL.     Basaltgesteine.   Bonn,  1870.  70. 

SANDBERGER.     N.  Jahrb.  f.  Min.  u.  Geol.   1870.  206. 

STRENG.     N.  Jahrb.  f.  Min.  u.  Geol.  1872.  385. 

GUMBEL.     D.  palaolith.  Eruptivgest.  d.  Fichtelgebirges.  Miinchen,  1874.  35. 

DATHE.     Zeitschr.  d.  deutsch.  geol.  Ges.  1874.  XXVI.   i. 

COHEN.     Reisen  in  Sudafrika.   Hamburg,  1875.  2.   Friedrichsen'sche  Jahresber. 

der  geograph.  Ges.     Comp.  Ref.  N.  Jahrb.  f.  Min.  u.  Geol.   1876.  213. 
v,  LASAULX      Verh.  d.  naturw.  Ver.  d.  preuss.  Rheinlande  u.  Westphal.    1878. 

XXXV. 

SAUER    N.  Jahrb.  f.  Min.  u.  Geol.   1879.  575. 
CH.  WHITMAN  CROSS.     Tsch.  Min.  u.  petr.  Mitth.  1881.  III.  401. 
CATHREIN.     Gr.  Zeitschr.  f.  Kryst.   1882.  244. 

Labradorite. 

VOGELSANG.     Archiv.  Neerland.   1868.   III. 

SCHRAUF.     Sitzungsber.  d.  Wien.  Akad.  d.  Wiss.   Dec.  1869.  LX.  Bd. 

STELZNER.     Berg-  und  Hiittenmann.  Zeig.  XXIX.   150. 

HAGGE.     N.  Jahrb.  f.  Min.  u.  Geol.   1871.  946. 

SCHUSTER.     Tsch.  Min.  u.  petr.  Mitth.  1881.   III.   183. 

Leucite. 

ZIRKEL.     Zeitschr.  d.  deutsch.  geol.  Ges.   1868.  97. 

—  Basaltgesteine.   Bonn,  1870. 

V.  RATH.     Monatsber.  d.  Berlin.  Akad.  d.  Wiss.  Aug.  1872. 


2O6       DETERMINATION  OF  ROCK-FORMING  MINERALS. 

KREUTZ.     Tsch.  Min.  u.  petr.  Mitth.   1884.  VI.   135. 

v.  CHRUSTSCHOFF.     Tsch.  Min.  u.  petr.  Mitth.  1884.  VI.  161. 

Liebenerite. 
ZIRKEL.     N.  Jahrb.  f.  Min.  u.  Geol.   1868.  719. 

Magnesite. 
ROSENBUSCH.     N.  Jahrb.  f.  Min.  u.  Geol.   1884.  I.   196. 

Magnetite. 

ZIRKEL.     Basaltgesteine.  Bonn,  1870.  67. 

VELAIN.     Descript.  geol.  d'Aden,  Reunion,  des  lies  St.  Paul  et  Amsterdam. 
Paris,  1877. 

Meionite. 

v.  RATH.     Zeitschr.  d.  deutsch.  geol.  Ges.   1866.  XVIII.  608,  626.  633. 
v.  KALKOWSKY.     Zeitschr.  d.  deutsch.  geol.  Ges.  1878.  XXX.  663. 

Melanite. 

FOUQUE.     Compt.  rend.   15  mars  1875. 

WICHMANN.      Pogg.  Ann.  f.  Phys.  u.  Chem.   1876.   CLVII.   282. 

KNOP.     Gr.  Zeitschr.  f.  Krystall.   1877.  58. 

Melilith. 

v.  RATH.     Zeitschr.  d.  deutsch.  geol.  Ges.   1866.  XVLII.   527. 
ZIRKEL.     Zeitschr.  d.  deutsch.  geol.  Ges.    1868.  XX.   118. 

—  Basaltgesteine.   Bonn,  1870.   77. 

HUSSAK.     Sitzungsber.  d.  Wien.  Akad.  d.  Wiss.  April  1878. 
STELZNER.     N.  Jahrb.  f.  Min.  u.  Geol.   1882.  II.  Beil.-Bd.  369. 

Meroxene  (Biotite). 

TSCHERMAK.     Sitzungsber.  der  Wien.  Akad.  der  Wiss.   1869.  May.  LIX. 

—  Gr.  Zeitschr.  f.  Kryst.   1878.   II.   18. 
ZIRKKL.     Basaltgesteine.   Bonn,  1870.   76. 

—  Ber.  d.  kgl.  sachs.  Ges.  d.  Wiss.  July  21,  1875. 

v.  KALKOWSKY.     Die  Gneissformation  d.  Eulengebirges.  Leipzig,  1878.  28. 

—  N.  Jahrb.  f.  Min.  u.  Geol.   1880.  I.  33. 


BIBLIOGRAPHY    TO  PART  II.  2O/ 

KISPATIC.  Tsch.  Min.  u.  petr.  Mitth.  1882.  IV.  127. 
WILLIAMS.  N.  Jahrb.  f.  Min.  u.  Geol.  1882.  II.  616. 
BECKER.  N.  Jahrb.  f.  Min.  u.  Geol.  1883.  II.  i. 

Microcline  (Microperthite). 

DES  CLOIZEAUX.     Ann.  de  chim.  et  phys.   1876.  9.  433. 
DATHE.     Zeitschr.  d.  deutsch.  geol.  Ges.   1877.  XXI.  274. 

—  Zeitschr.  d.  deutsch.  geol.  Ges.    1882.  XXXIV.   12. 
M.  LEVY.     Bull.  d.  la  societ.  miner.     No.  5.   1879. 
BECKE.     Tsch.  Min.  u.  petr.  Mitth.   1882.   IV.   196. 
KOLLER.     Tsch.  Min.  u.  petr.  Mitth.   1883.  V.  218. 
KLOOS.     N.  Jahrb.  f.  Min.  u.  Geol.   1884.   II.  87. 

Muscovite  (Sericite). 

LOSSEN.     Zeitschr.  d.  deutsch.  geol.  Ges.   1867.  XIX.  509. 

WICHMANN.     Verh.  des  naturf.  Ver.  f.  d.  Rheinlande.  XXXIV.  5.  F.  4.  Bd. 

TSCHERMAK.     Gr.  Zeitschr.  f.  Kryst.   1878.  40. 

v.  LASAULX,     N.  Jahrb.  f.  min.  u.  Geol.   1872.  851. 

v.  GRODDECK.     Jahrb.  d.  kk.  geol.  R.-Anst.   Wien,  1883.  397. 

Nepheline. 

ZIRKEL.     Pogg.  Ann.  f.  Phys   u.  Chem.   1867.  298. 

-  N.  Jahrb.  f.  Min.  u.  Geol.   1868.  697. 

—  Basaltgesteine.   Bonn,  1870. 

ROSENBUSCH.     Nephelinit  v.  Katzenbuckel.   Freiburg,   1869     Ref.  N.  Jahrb.  f. 

Min.  u.  Geol.   1869.  485. 
BORICKY.     Archiv.  d.  naturw.  Landesdurchforsch.  Bohmens.    Prag,    1874.   Die 

Phonolithe.  8. 

Nosean. 

v.  RATH.     Zeitschr.  d.  deutsch.  geol.  Ges.   1862.  XIV.  663. 
ZIRKEL.     Pogg.  Ann.  f.  Phys.  u.  Chem.  1867.  CXXXI.  312. 
ROSENBUSCH.   Nephel.  v.  Katzenbuckel.   1869.  35. 

BoiucKY.     Archiv.  d.  naturw.  Landesdurchforsch.  Bohmens.    Prag,  1873.    Die 
Basaltgesteine.   27. 

-  The  same.   1874.  Die  Phonolithe.   10. 

Oligoclase. 

ZIRKEL.  Zeitschr.  d.  deutsch.  geol.  Ges.  1867.  XIX.  100. 
M.  SCHUSTER.  Tsch.  Min.  u.  petr.  Mitth.  1881.  III.  164. 
MUGGE.  N.  Jahrb.  f.  Min.  u.  Geol.  1881.  II.  107. 


2O8       DETERMINATION  OF  ROCK-FORMING  MINERALS. 

Oligoclasalbite. 
SCHUSTER.     Tsch.  Min.  u.  petr.  Mitth.   1881.  III.   159. 

Olivine. 

TSCHERMAK.     Sitzungsber.  d.  Wien.  Akad.  d.  Wiss.   1866.   LIII.   260. 

—  Sitzungsber.  d.  Wien.  Akad.  d.  Wiss.   1867.  July.  LVI. 
ZIRKEL.     Basaltgesteine.   Bonn,  1870.  55. 

—  Zeitschr.  d.  deutsch.  geol.  Ges.   1871.   59. 

HAGGE.     Ueber  Gabbro.   Kiel,  1871.   N.  Jahrb.  f.  Min.  u.  Geol.   1871.  946. 

ROSENBUSCH.     N.  Jahrb.  f.  Min.  u.  Geol.   1872.   59. 

DATHE.     N.  Jahrb.  f.  Min.  u.  Geol.   1876.  225,  337. 

PENCK.     Zeitschr.  d.  deutsch.  geol.  Ges.   1878.  XXX.  97. 

BROGGER.     N.  Jahrb.  f.  Min.  u.  Geol.   1880.  II.   187. 

COHEN.     N.  Jahrb.  f.  Min.  u.  Geol.   1880.   II.  31,  52. 

v.  FOULLON.     Tsch.  Min.  u.  petr.  Mitth.   1880.   II.   181. 

BECKER.     Zeitschr.  d.  deutsch.  geol.  Ges.   1881.  XXXIII.  31. 

BECKE.     Tsch.  Min.  u.  petr.  Mitth.   1882.   IV.  322,  355,  450. 

—  Tsch.  Min.  u.  petr.  Mitth.   1882.  V.   163. 
SCHRAUF.     Gr.  Zeitschr.  f.  Kryst.   1882.   321. 
KREUTZ.     Tsch.  Min.  u.  petr.  Mitth.   1884.  VI.   142. 

Opal. 

M.  SCHULTZE.      Verh.   d.   naturf.  Ver.   d.   preussischen    Rheinlande   u.  Wcst- 

phalens.    1861.   69. 

G.  ROSE.     Monatsber.  d.  Berlin.  Akad.  d.  Wiss.   1869.  449. 
BEHRENS.     Sitzungsber.  d.  Wien.  Akad.  d.  Wiss.   1871.   LXIV.  I.  Abth. 
VELAIN.     Descript.  geolog.  d'Aden,  Reunion  .   .  .   Paris,  1877.  32,  322. 
KISPATIC.     Tsch.  Min.  u.  petr.  Mitth.   1882.  IV.   122. 

Orthoclase  (Sanidine). 

REUSCH.     Pogg.  Ann.  f.  Phys.  u.  Chem.    1862.  CXVI.  392,  and  1863.  CXVIII. 

256. 
ZIRKEL.     Pogg.  Ann.  f.  Phys.  u.  Chem.   1867.  CXXXI.  300. 

—  Sitzungsber.  d.  Wien.  Akad.  d.  Wiss.   1863.  XLVII.  237,  246. 

—  N.  Jahrb.  f.  Min.  u.  Geol.   1866.  775. 

—  Zeitschr.  d.  deutsch.  geol.  Ges.   1867.  XIX.   87. 
LASPEYRES.     Zeitschr.  d.  deutsch.  geol.  Ges.   1864.  XVI.  392. 

S.  WEISS.  Beitr.  z.  Kenntn.  d.  Feldspathbilduner.  Haarlem,  1866. 
ROSENBUSCH.  Verh.  d.  Naturf.  Ver.  Freiburg.  VI.  i,  95,  98,  103. 
STRENG.  N.  Jahrb.  f.  Min.  u.  Geol.  1871.  598. 


BIBLIOGRAPHY   TO  PART  II.  209 

Ottrelite. 

V.  LASAULX.     N.  Jabrb.  f.  Min.  u.  Geol.   1872.   849. 
TSCHERMAK  u.  Sii'dcz.     Gr.  Zeitschr.  f.  Kryst.  1879.   5°9- 
BKCKE.     Tsch.  Min.  u.  petr.  Mitth.    1878.   I.  270. 

RKXARD  et  VALLEE  POUSSIN.     Ann.  de  la  Soc.  geol.  Belgique.    VI.    Mem.    51. 
N.  Jahrb.  f.  Min.  u.  Geol.   1880.   II.   149. 

Perowskite. 

BOKICKY.     Sitzungsber.  der  math.-naturw.   Classe  d.  k.  bohm.  Ges.  d.  Wiss. 

1876.  Comp.  Ref.  N.  Jahrb.  f.  Min.  u.  Geol.   1877.   539- 

HUSSAK.     Sitzungsber.  d.  Wien.  Akad.  d.  Wiss.  math.-nat.  Classe.  April  1878. 
STELZNER.     N.  Jahrb.  f.  Min.  u.  Geol.   1882.  II.  Beil.-Bd.  390. 

Phlogopite. 
TSCHERMAK.     Gr.  Zeitschr.  f.  Kryst.  1878.  33. 

Picotite. 

ZIRKEL.     Basaltgesteine.   Bonn,  1870.  97. 

STELZNER.     N.  Jahrb.  f.  Min.  u.  Geol.   1882.   II.  Beil.-Bd.  393. 

Finite  (and  other  decomposition-products  of  Cordierite). 
WICHMANN.     Zeitschr.  d.  deutsch.  geol.  Ges.   1874.  XXVI.  675. 

Flagioclase. 

TSCHERMAK.     Sitzungsber.  d.  Wien.  Akad.  d.  Wiss.  L.  Dec.  1864. 
WEISS.      Beitr.  z.  Kenntn.  d.  Feldspathbildung.   Haarlem,  1866. 
ROSE.     Zeitschr.  d.  deutsch.  geol.  Ges.  XIX.   1867.  289. 
STELZNER.     N.  Jahrb.  f.  Min.  u.  Geol.   1870.  784. 
ZIRKEL.     Zeitschr.  d.  deutsch.  geol.  Ges.   1871.  XXIII.     43,  59,  94. 
-  Basaltgesteine.   Bonn,  1870.  28. 
HAGGE.     Ueber  Gabbro.   Kiel,  1871. 
STRENG.     N.  Jahrb.  f.  Min.  u.  Geol.   1871.  598,  715. 
COHEN.     N.  Jahrb.  f.  Min.  u.  Geol.   1874.  460. 
v.  RATH.     Monatsber.  d.  Berlin.  Akad.  d.  Wiss.  24.  Feb.  1876. 
ROSENBUSCH.     Verh.  d.  naturforsch.  Ges.   Freiburg  i.  Br.  VI.   I,  77. 
PENCK.     Zeitschr.  d.  deutsch.  geol.  Ges.   1878.  XXX.  97. 
PFAFF.     Sitzungsber.  d.  phys.-med.  Societ.  z.  Erlangen.  1878. 
SCHUSTER.     Tsch.  Mm.  u.  petr.  Mitth.   iSSi.   III.   117. 
—  Tsch.  Min.  u.  petr.  Mitth.   1882.  V.   189. 


210      DETERMINATION  OF  ROCK-FORMING  MINERALS. 

HOEPFNER.     N.  Jahrb.  f.  Min.  u.  Geol.   1881.   II.   164. 
BECKE.     Tsch.  Min.  u.  petr.  Mitth.   1882.  IV.  253. 
KLOCKMANN.     Zeitschr.  d.  deutsch.  geol.  Ges.   1882.  373. 
v.  WERVEKE.     N.  Jahrb.  f.  Min.  u.  Geol.  1883.   II.  97. 
KREUTZ.     Tsch.  Min.  u.  petr.  Mitth.  1884.  VI.  137. 

Pleonaste. 
TELLER  u.  JOHN.     Jahrb.  d.  kk.  geol.  R.-Anst.  Wien,  1882.  XXXII.  589. 

Protobastite  (Diaclasite). 

TSCHERMAK.     Tsch.  Min.  Mitth.   1871.   i.   Heft.  20. 
STRENG.     N.  Jahrb.  f.  Min.  u.  Geol.  1872.  273.  Anm.  2. 

Pyrope. 

DOELTER.     Tsch.  Min.  Mitth.   1873.   J3- 

SCHRAUF.     Gr.  Zeitschr.  f.  Kryst.   1882.  321  and  1884.  II.  21. 

Quartz. 

H.  CLIFTON  SORBY.     Quart.  Journ.  geol.  Soc.  Nov.  1858.  XIV.  453. 
ZIRKEL.     N.  Jahrb.  f.  Min.  u.  GeoL   1868.   711. 

—  Pogg.  Ann.  f.  Phys.  u.  Chem.   1871.   CXXXXIV.  324. 
ROSENBUSCH.     Reise  n.  Sudbrasilien.   Freiburg  i.  Br.,  1870. 
BEHRENS.     N.  Jahrb.  f.  Min.  u.  Geol.   1871.  460. 

LEHMANN.    Verh.  d.  niederrhein.  Ges  f.  Nat.  u.  Heilkunde.  Bonn,  1874.  XXXI. 

—  Verh.  d.  naturhist.  Ver.  d.  preuss.  Rheinlande  u.  Westphalens.  1874.  XXXIV. 
v.  CHRUSTSCHOFF.     Tsch.  Min.  u.  petr.  Mitth.   1882.   IV.  473. 

BoftlCKY.     Archiv  d.  naturw.  Landesdurchf.  Bohmens.   1882.   IV.  No.  4.   12. 

Ripidolite  (Chlorite,  Helminth). 
O.  MEYER.     Zeitschr.  d.  deutsch.  geol.  Ges.   1878.  XXX.  i,  24. 

Rubellan. 
HOLLRUNG.     Tsch.  Min.  u.  petr.  Mitth.  1883.  V.  304. 

Rutile. 

SAUER.     N.  Jahrb.  f.  Min.  u.  Geol.  1879.  569. 

—  N.  Jahrb.  f.  Min.  u.  Geol.   1880.   I.  227,  279. 

v.  WERVEKE.     N.  Jahrb.  f.  Min.  u.  Geol.   1880.  II.  281. 


BIBLIOGRAPHY   TO  PART  II.  211 

CATHREIN.     N.  Jahrb.  f.  Min.  u.  Geol.   1881.  I.   169. 

-Gr.  Zeitschr.  F.  Kryst.   1883.  VIII.  321. 

H.  GYLLING.     N.  Jahrb.  f.  Min.  u.  Geol.   1882.  I.   163. 

PICHLER  u.  BLAAS.     Tsch.  Min.  u.  petr.  Mitth.   1882.  IV.  513. 

SANDBERGER.     N.  Jahrb.  f.  Min.  u.  Geol.  1882.  II.  192. 

v.  LASAULX.     Gr.  Zeitschr.  f.  Kryst.   1883.  VIII.  54. 

Serpentine  (comp.  Olivine). 

WEBSKY.     Zeitschr.  d.  deutsch.  geol.  Ges.  1858.  277. 
WEISS.     Pogg.  Ann.  f.  Phys.  u.  Chem.   1863.  CXIX.  458. 
TSCHERMAK.     Sitzungsber.  d.  Wien.  Akad.  d.  Wiss.  LVI.  July  1867. 
ZIRKEL.     N.  Jahrb.  f.  Min.  u.  Geol.   1870.  829. 
J.  ROTH.  Abhandl.  d.  Berlin.  Akademie  d.  Wiss.   1869. 
DRASCHE.     Tsch.  Min.  Mitth.  1871.  i. 
WEIGAND.     Tsch.  Min.  Mitth.  1875.  183. 
DATHE.     N.  Jahrb.  f.  Min.  u.  Geol.   1876.  225,  337. 
LEMBERG.     Zeitschr.  d.  deutsch.  geol.  Ges.   1877.  XXX.  457. 
BECKE.     Tsch.  Min.  u.  petr.  Mitth.   1878.  I.  459,  470. 
-  Tsch.  Minn.  u.  petr.  Mitth.   1882.  IV.  322. 
HARE.     Serpentin  von  Reichenstein.   In.-Diss.   Breslau,  1879. 
HUSSAK.     Tsch.  Min.  u.  petr.  Mitth.   1882.  V.  6r. 
SCHRAUF.     Gr.  Zeitschr.  f.  Kryst.   1882.  321. 
SCHULZE.     Zeitschr.  d.  deutsch.  geol.  Ges.   1883.  XXXV.  433. 

Sillimanite. 

v.  KALKOWSKY.     Die  Gneissform.  d.  Eulengebirges.  Leipzig,  1878. 
SCHUMACHER.     Zeitschr.  d.  deutsch.  geol.  Ges.  1878.  427. 
BECKE.     Tsch.  Min.  u.  petr.  Mitth.  1882.  IV.  189. 

Scapolite. 

MICHEL  LEVY.     Bull.  Soc.  miner.  France.   1878.  No.  3  and  5. 

BECKE.     Tscherm.  Min.  u.  petr.  Mitth.   1882.  IV.  369. 

TORNEBOHM.     Geol.  Foren.  i  Stockholm  Forhandl.    VI.    185.    Comp.  Ref.  N. 

Jahrb.  f.  Min.  u.  Geol.   1883.   I.  245. 
CATHREIN.     G.  Zeitschr.  f.  Kryst.  1884.  IX.  378. 

Sodalite. 

v.  RATH.     Zeitschr.  d.  deutsch.  geol.  Ges.   1866.  620. 

—  Verh.  d.  niederrhein.  Ges.  f.  Nat.  u.  Heilkunde.   1876.  82. 

VRBA.     Sitzungsber.  d.  Wien.  Akad.  d.  Wiss.  LXIX.  Feb.  1874. 


212       DETERMINATION  OF  ROCK-FORMING  MINERALS. 

v.  KALKOWSKY.     Zeitschr.  d.  deutsch.  geol.  Ges.   1878.  663. 
v.  WERVEKE.     N.  Jahrb.  f.  Min.  u.  Geol.   1880.  II.   141. 
KOCH.     N.  Jahrb.  f.  Min.  u.  Geol.   1881.   I.  Beil.-Bd.   149. 

Staurolite. 

PETERS  u.  MALY.     Sitzungsber.  d.  Wien.  Akad.  d.  Wiss.  LVII.   1868.   15. 
v.  LASAULX.     Tsch.  Min.  u.  petr.  Mitth.   1872.   III.   173,  and  N.  Jahrb.  f.  Min 

u.  Geol.   1872.  838. 
O.  MEYER.     Zeitschr.  d.  deutsch.  geol.  Ges.   1878.  XXX.  i. 

Talc. 

v.  LASAULX.     N.  Jahrb.  f.  Min.  u.  Geol.   1872.  823. 
TSCHERMAK.     Tsch.  Min.  Mitth.   1876.  I.  65. 

Titanite. 

ZIRKEL.     Zeitschr.  d.  deutsch.  geol.  Ges.   1859.  XL   522,  526. 

—  Pogg.  Ann.  f.  Phys.  u.  Chem.   1867.  CXXXI.  325. 

v.  RATH.     Zeitschr.  d.  deutsch.  geol.  Ges.   1862.  XIV.  665. 

—  Zeitschr.  d.  deutsch.  Geol.  Ges.   1864.  XVI.  256. 
GROTH.     N.  Jahrb.  F.  Min.  u.  Geol.   1866.  46. 

v.  LASAULX.     N.  Jahrb.  f.  Min.  u.  Geol.   1872.  362. 
v.  WERVEKE.     N.  Jahrb.  f.  Min.  u.  Geol.   1880.   II.   159. 
MANN.     N.  Jahrb.  f.  Min.  u.  Geol.   1882.  II.  200. 
DILLER.     N.  Jahrb.  f.  Min.  u.  Geol.   1883.   I.   187. 

Titaniferous  Magnetite. 

v.  WERVEKE.     N.  Jahrb.  f.  Min.  u.  Geol.  1880.   II.   141. 
CATHREIN.     Gr.  Zeitschr.  f.  Kryst.   1883.  VIII.  321. 

Tremolite  (Grammatite). 

TSCHERMAK.     Tsch.  Min.  Mitth.   1871.  37.  and  1876.  65. 
BECKE.     Tsch.  Min.  u.  petr.  Mitth.   1882.   IV.  338. 

Tridymite. 

ZIRKEL.     Pogg.  Ann.  f.  Phys.  u.  Chem.   1870.  CXL.  492. 
v.  LASAULX.     N.  Jahrb.  f.  Min.  u.  Geol.   1869.  66. 

—  Gr.  Zeitschr.  f.  Kryst.   1878.   II.  254. 
STRENG.     Tsch.  Min.  Mitth.  1871.  47. 

—  N.  Jahrb.  f.  Min.  u.  Geol.   1872.  266. 


BIBLIOGRAPHY    TO  PART  II.  21$ 

ROSENBUSCH.     Verhandl.  d.  naturf.  Ges.  Freiburg  i.  Br.   1873.  VI.    I.  Hft.   96. 
SCHUSTER.    Tsch.  Min.  u.  petr.  Mitth.   1878.  71. 

Tourmaline. 

ZIRKEL.     N.  Jahrb.  f.  Min.  u.  Geol.   1875.  628. 

TORNEBOHM.     Geol.  Foren.  i.  Stockholm  Forhandl.   1876.  III.  218. 

MEYER.     Zeitschr.  d.  deutsch.  geol.  Ges.  1878.  XXX.  i,  24. 

WICHMANN.     N.  Jahrb.  f.  Min.  u.  Geol.   1880.  II.  294. 

DATHE.     Zeitschr.  d.  deutsch.  geol.  Ges.  1882.  XXXIV.  12. 

PICHLER  u.  BLAAS.     Tsch.  Min.  u.  petr.  Mitth.  1882.  IV.  512. 


Uralite. 

G.  ROSE.     Reise  nach  dem  Ural.  II.  371. 
BECKE.     Tsch.  Min.  u.  petr.  Mitth.  1882.  V.  157. 

Viridite  (Delessite,  Chlorophaeite). 

VOGELSANG.     Zeitschr.  d.  deutsch.  geol.  Ges.  1872.  XXIV.  529. 

KOSMANN.     Verh.  d.  naturw.  Ver.  d.  preuss.  Rheinlande  u.  Westph.  XXV.  239. 

and  289. 
TSCHERMAK.     Die  Porphyrgesteine  Oesterreichs.  Wien,  1869.  42,  66,  134. 

—  Tsch.  Min.  Mitth.   1872.  112. 

Wollastonite. 

FOUQUE.     Compt.  rend.  15  Mar.  1875. 

LAGORIO.     Andesite  d.  Kaukasus.    Dorpat,   1878.    Ref.  N.  Jahrb.  f.  Min.   u. 

Geol.  1880.  I.  209. 

CH.  WHITMAN  CROSS.     Tsch.  Min.  u.  petr.  Mitth.   1881.  III.  373. 
T6RNEBOHM.     Geol.  Foren.  i.  Stockholm  Forh.   1883.  VI.  No.  12.  542.  Comp. 

Ref.  N.  Jahrb.  f.  Min.  u.  Geol.  1884.  I.  230. 

Zeolite  (Analcime). 

ROSENBUSCH.     Nephelinit  v.  Katzenbuckel.  Freiburg  i.  Br.,  1869. 
KLOOS.     N.  Jahrb.  i.  Min.  u.  Geol.  1884.  III.  Beil.-Bd.  37. 

Zircon. 

SANDBERGER.     Wurzburger  nat.  Zeitschr.   1866/67.  VI.  128  and  1883. 

—  Zeitschr.  d.  deutsch.  geol.  Ges.  '1883.  XXXV.  193. 

—  N.  Jahrb.  f.  Min.  u.  Geol.   1881.   I.  258. 


214      DETERMINATION  OF  ROCK-FORMING  MINERALS. 

ZIRKEL.     N.  Jahrb.  f.  Min.  u.  Geol.   1875.  628. 

—  N.  Jahrb.  f.  Min.  u.  Geol.   1880.   I.  89. 

TORNEBOHM.     Geol.  Fohren.  i  Stockholm  Forhandling.  1876.   III.  No.  34.  and 

N.  Jahrb.  f.  Min.  u.  Geol.   1877.  97. 

MICHEL  LEVY.     Bull.  Soc.  mineral.  France.   1877.  No.  5.  77. 
ROSENBUSCH.     Sulla  presenza  dello  zircone  nelle  roccie.  Atti  d.  R.  Accadern.  d. 

Science.  Torino  1881.  Vol.  XVI. 
BECKE.     Tsch.  Min.  u.  petr.  Mitth.  1882.  IV.  204. 
FLETCHER.     Gr.  Zeitschr.  f.  Kryst.  1882.  80. 
NESSIG.     Zeitschr.  d.  deutsch.  geol.  Ges.  1883.  XXXV.  118. 
v.  CHRUSTSCHOFF.     Tsch.  Min.  u.  petr.  Mitth.   1884.  VI.   172. 

Zoisite. 

RIESS.     Tsch.  Min.  u.  petr.  Mitth.   1878.   I.   188. 

BECKE.     Tsch.  Min.  u.  petr.  Mitth.  1878.  I.  249.  and  1882.  IV.  312. 


EXPLANATIONS  OF  CUTS  ACCOMPANYING 
PART  II. 


FIG. 


PAGE 

51  ILMENITE.     Grain,  partly  decomposed  into  leucoxene,  with  undecom- 

posed  earthy  filaments  interlaminated m 

52  OPAL.     As  filling  of  a  cavity,  in  concentric  layers,    inclosing  small 

groups  of  tridymite  tablets.     (After  Fouque.) 112 

53  HAUYN.     Cross  section  with  opacitic  border  and  vitreous  inclosures; 

penetrated  by  a  network  of  black  lines  crossing  each  other  at  right 
angles IJ5 

54  a.  MELANITE  cross  section,  zonally  developed 117 

b.  ALMANDINE  GRAIN,    with   inclosures  of  quartz-grains  ;   traversed 

by  irregular  cleavage  fissures 117 

55  PYROPE  GRAIN  (P)  with  border  of  so-called  kelyphite  (K).     From  the 

serpentine  (S)  from  Kremse,  Bohemian  forest.  On  the  serpentine 
portion  (5)  showing  the  "  mesh  -structure"  is  a  thin  layer  of  fresh 
olivine  grains,  followed  by  the  fibrous  metamorphosed  zone  (fC)  of 
Pyrope;  this  has  been  called  kelyphite  by  Schrauf,  and  is  a  "  pyro- 
gene"  product,  although  regarded  by  others  as  an  "  hydatogene"  pro- 
duct, and  has  been  regarded  as  allied  to  an  augitic  mineral 117 

56  PEROWSKITE  GRAINS  in  the  so-called  "hacked"  figures.    (After  Stelz- 

ner.) 120 

57  LEUCITE  cross-section  in  polarized   light,  showing  the  polysynthetic 

striation.     (After  Zirkel.) 122 

58  Cross  sections  of   small   LEUCITE   crystals  and   grains   (constituents 

II.  order),  with  vitreous  inclosures  regularly  distributed 123 

59  RUTILE  CRYSTAL.       Knee-,  heart-shaped,  and  polysynthetic   twins. 

(After  Reusch.) 122 

60  ZIRCON  CRYSTALS.     (After  Fouque.) 124 

61  SCAPOLITE  cross-section,  at  right  angles  to  the  chief  axis,  with  rectan- 

gular cleavage 124 


2l6       DETERMINATION  OF  ROCK-FORMING  MINERALS. 

FIG  PAGE 

62  MELILITE.     a.  Cross-sections  parallel  to  the  chief  axis.    The  upper 

shows  a  separation  into  fine  fibres  and  cleavage-fissures  parallel  oP\ 
the  under,  the  so-called  "  pflock-structure,"  pear-shaped  and  spin- 
dle-shaped canals  originating  from  the  face  oP,  which  appear  as  a 
small  circle  in  sections  (parallel)  oP  (Fig.  62,  b).  Fig.  62,  c,  shows  a 
larger  cross-section  of  an  irregular  grain,  wherein  small  leucite 
grains  are  developed,  (a  and  b  after  Stelzner.) 127 

63  QUARTZ,     a — d  are   cross-sections   of   the    conchoidal    crystal  skele- 

ton, which  occur  interpenetrated  with  orthoclase  "  micropegmatitic." 
a.  Section  parallel  to  the  chief  axis.  b.  Section  parallel  to  the 
base.  c.  Section  at  right  angles  to  the  prismatic  edges,  d.  Section 
inclined  to  the  same.  (After  Fouque.)  e.  Cross  section  of  an  ortho- 
clase wherein  quartz  is  developed  micropegmatitic 129 

64  TRIDYMITE.     Crystal  groups  of  thin  hexagonal  tablets  overlapping 

each  other  like  roof-tiles.     (After  Fouque.) 131 

65  CALCITE  GRAIN,  with  rhombohedral  cleavage  and  twinning  striations. 

After  —  \R 132 

66  NEPHELINE.     a.  Transverse  section,     b.  Longitudinal  section,  with 

augitic  inqlosures  zonally  distributed 134 

67  APATITE,     a.   Transverse    section,      b.   Longitudinal    section,    with 

cleavage-fissures  and  acicular  inclosures  parallel  to  the  base 137 

68  TOURMALINE,     a.   Longitudinal  section,      b.  Transverse  section   zo- 

nally  developed 138 

69  TOURMALINE  CRYSTAL.     (After  Reusch.) 138 

70  OLIVINE  cross-section  in  different  degrees  of  decomposition,     a.  With 

undecomposed  centre,  b.  "  Serpentinized  "  only  on  the  edges  an4 
cleavage-fissures 141 

71  OLIVINE  cross-section,     a.   Cross  section  parallel  oP.     b.   Cross-sec- 

tion parallel  oo  p  o>.     (After  Fouque.) 140 

72  SILLIMANITE.     a.  Transverse  section,     b.   Long,  broken  needle,  with 

transverse  fissures 142 

73  STAUROLITE.     Twin  with  inclosures  of  quartz  granules  ;  the  -f-  sign 

annexed  indicates  the  position  of  the  directions  of  vibration  in  the 
individual  which  is  hatched  142 

74  ENSTATITE  and  BRONZITE  transverse  sections,     a.    Optical  orienta- 

tion according  to  Tschermak's  position,  b.  According  to  G.  v. 
Rath's  position 144 

75  ENSTATITE  longitudinal  section,  with  the  cleavage  fissures  parallel  to 

the  vertical  axis  partially  decomposed  into  bastite , .     145 


EXPLANATIONS  OF  CUTS  ACCOMPANYING  PART  II.   21? 

PIG.  PAGE 

76  ANDALUSITE  cross-sections,     a.  Transverse  section  with  rectangular 

cleavage-cracks,  and  opaque  granules  distributed  centrally  and  in  a 
cross-shape.  (Similar  to  chiastolite.) 153 

77  CORDIERITE  GRAIN,  with  a   fibrous   decomposition  on  the  cleavage- 

cracks,  with  inclosures  of  sillimanite  needles 155 

78  Transverse  section  of  a  twinned  cordierite  crystal.     The  apparently 

hexagonal  crystal,  composed  of  three  individuals,  divides  into  six 
fields  in  polarized  light,  two  of  which  lying  opposite  extinguish  to- 
gether ;  the  position  of  the  directions  of  vibration  is  designated  by 
a  mark 154 

79  ZOISITE  cross-sections,     a.  Transverse  section,     b.   Longitudinal  sec- 

tion, showing  cleavage  fissures  and  fluid  inclosures  arranged  in  a 
series 155 

80  BIOTITE  leaflet,  parallel  oP\  the  outer  portions  are  decomposed  into 

chlorite  and  contain  earthy  granules  and  epidote  needles  ;  the  irregu- 
larly defined  kernel  is  fresh 156 

81  BIOTITE   longitudinal    section,    showing  cleavage-cracks  parallel  oP 

and  inclosures  of  calcite  lenses 156 

82  OTTRELITE.     Section  at  right  angles  to  oP,  twinned  polysynthetically 

after  oP.  The  annexed  -f-  indicates  the  position  of  the  directions  of 
vibration 164 

83  SANIDINE  cross-sections,     a  =  parallel  oP  or  oo  P  oo.     b  =  Carlsbad 

twin,     c  =  Baveno  twin,     d  =  parallel  co^Pco  with  a  combination 

of  oP.  co  f  oo  .  2P  co.     e  —  parallel  ooP  oo.     (After  Rosenbusch.). . .     166 

84  AUGITE  cross-section,     a.   At    right  angles  to    the  vertical   axis.     b. 

Parallel  to  the  orthopinacoid.  c.  Parallel  tx>  the  clinopinacoid. 
(After  Fouque.) 168 

85  URALITE  cross-section.     The   seconary   hornblende   is   partially   de- 

veloped over  the  augite,  with  a  twin  lamella  after  oo/'oo.  (After 
Becke.) 175 

86  HORNBLENDE    cross  section.       a.  Transverse    section.        b.  Parallel 

coiPoo.     c.   Parallel  co  P  oo.     (After  Fouque.) 172 

87  EPIDOTE.     Optical  orientation.     (After  Klein  and  v.  Lasaulx.)     Opt. 

A.  =  optic  axes  (for  red  and  green),  I.  a  first  negative  middle  line, 
II.  c  =  second  middle  line,  b  =  b  optic  normals,  a.  Clinodiagonal 
and  one  direction  of  cleavage,  c.  Vertical  axis 176 

88  EPIDOTE  twin  after  oo  P  o>.     (After  Reusch.) 176 

89  EPIDOTE  CRYSTAL.     (After  Reusch.) 176 


218       DETERMINATION  OF  ROCK-FORMING  MINERALS. 

FIG.  PAGE 

90  TITANITE.     Cross-section  of  crystals  and  grains  ;  simple  individuals 

and  twins  after  oP 176 

91  MICROCLINE.     Section  parallel  oP  shows  the  latticed  twinning  stria- 

tions  and  lenticular  albite  developed  within,  with  polysynthetic 
striations  also  180 

92  MICROCLINE  from  Lampersdorf,  Silesia.     (After  A.  Beutell.)     Section 

parallel  0/*.  The  microcline  is  in  part  homogeneous,  in  part  shows 
the  latticed  structure  ;  the  larger  albite  bands  run  parallel  to  the 
edge  oP  :  P  o°  and  show  fine  twinnings  striation  parallel  oP :  oo  P  <x> . .  180 

93  MICROPERTHITE.     a  =  section  parallel   to  the  separation-plane  cor- 

responding <aP  <x>,  shows  a  peculiar  network  composed  of  filaments 
refracting  light  powerfully,  crossing  each  other  at  right  angles. 
b  —  section  parallel  oP,  c  parallel  co/  co,  both  with  entered  poly- 
synthetically  twinned  albite  lamellae.  (After  Becke.) 180 

94  Plagioclase  crystal  showing  the  position  of  the  obtuse  edge  P/ 'M, 

and  the  bearing  of  the  directions  of  extinction  toward  them.     (After 

Schuster.) 182 

195  PLAGIOCLASE.     Cross-section  parallel  M(<x>P<x>).     Right  longitudinal 
plane  (oo  j*5  oo)  of  a  crystal  correctly  oriented.     (Compare  Fig.  94.) 

The  obtuse  edge  P  :  M  lies  above 182 

96  PLAGIOCLASE.     Cross-section  parallel  P(oP}.     Upper   terminal  plane 

(oP)  of  an  oriented  crystal;  the  obtuse  edge  P  :  Miles  to  the  right.     182 
97-101*5.     Interference-figures  of  PLAGIOCLASE  on  cleavage-leaflets  par- 
allel M  and  P.     They  have  reference  to  the  upper  oP-  and  right 
oo  P  co-planes  of  an  oriented  crystal  (Fig.  94),  and  are  all  in  the  same 
position  as  Figs.  95  and  96. 

Fig.     97     Albite,  parallel  M( oo  P  oo) 182 

98     Oligoclase,  parallel  M( <x>  P  oo)  184 

99#  Labrador,  parallel  M(<x>  P  oo) 186 

99^  Labrador,  parallel  P(oP) 186 

Bytownite,  parallel  M(<x>  P  oo) 186 

Bytownite,  parallel  P(oF) 186 

Anorthite,  parallel  M(<x>  P  oo) 188 

Anorthite,  parallel  P(oP) 188 

(Figs.  95-101  after  Schuster.) 

102  ANDESINE.     Cross-section  parallel  oP.     Zonally  developed.     (After 

Becke. ) 185 

103  AGGREGATES  of  acicular  zeolite  crystals  and  concentric-conchoidal  car- 

bonates as  cavity-deposits 195 


CUTS  ACCOMPANYING  PART  II. 


FIG.  51. 


FIG.  52. 


220      DETERMINATION  OF  ROCK-FORMING  MINERALS. 
CUTS  ACCOMPANYING   PART   \\.-Continu.d. 


FIG.  56. 


FIG. 


59- 


~~~~~ 


FIG. 


FIG.  58. 


FIG.  61. 


CUTS  ACCOMPANYING  PART  II. 
CUTS  ACCOMPANYING  PART   \\.-Continued. 


221 


fPIP 


FIG.  63. 


FIG.  64. 


FIG.  65. 


a  6 

FIG.  66. 


FIG.  67. 


a  /» 

FIG.  68. 


FIG.  69. 


FIG.  70. 


222       DETERMINATION  OF  ROCK-FORMING  MINERALS. 
CUTS  ACCOMPANYING   PART   \\.— Continued. 


oP 


A 


I  of 


FIG.  71. 


FIG.  72. 


FIG.  74. 


FIG.  73. 


FIG.  75. 


FIG.  76. 


FIG.  77 


CUTS  ACCOMPANYING  PART  II. 


CUTS  ACCOMPANYING  PART   II.- Continued. 


UNJ  |  223 


FIG.  84. 


224       DETERMINATION  OF  ROCK-FORMING  MINERALS. 


CUTS   ACCOMPANYING   PART   \\.-Continued. 


FIG.  85. 


FIG.  86. 


FIG.  87. 


FIG. 


FIG.  89. 


CUTS  ACCOMPANYING  PART  II. 
CUTS   ACCOMPANYING  PART  II.— Continued. 


22$ 


FIG.  93. 


FIG.  94. 


226       DETERMINATION  OF  ROCK-FORMING  MINERALS. 


CUTS  ACCOMPANYING  PART   II.— Continued. 


FIG.  95. 


FIG.  96. 


FIG.  97. 


FIG.  98. 


FIG.  99  a. 


FIG.  99  b. 


CUTS  ACCOMPANYING  PART  II,  22J 

CUTS   ACCOMPANYING  PART  \\.-Concludcd. 


FIG.  ioo  a. 


FlG.  100  £. 


FIG.  ioi  a. 


FIG.  ioi  6. 


FIG.  102. 


FIG.  103. 


INDEX. 


PAGE 

Aegerine 197 

Aggregate 188 

Acinite ..  172,  197 

Actinolite 174,  197 

Albite     182,198 

Almandine 116,  198 

Ammonium-magnesium  Phosphate 62 

Amorphous  Minerals 112 

— ,  Behavior  of,  in  polarized  hght.iy,  30, 

106 

Analcime 118,  198 

Analyzer 8 

Anatase 124 

Andalusite 152,  198 

Andesine 184,  198 

Anisotrope 106 

Anomite     158,  199 

Anorthite 188,  199 

Anthophyllite 22,146,  199 

Apatite 53,  i3&  '99 

Arragonite 194 

Arfvedsonite 174,  199 

Augite,  Ordinary  and  basaltic 168,  199 

optical  orientation  of 24,  28 

shell-formed  structure  of 91,  92 

cleavage  of 84 

interpenetrations  of 93 

twins 41 

Axes  of  elasticity,  Determination  of  the 

position  of 26 

Axial  plane.  Determination  of  the  posi- 
tion of  the  optic 33 

Axial  colors,  Determination  of  the 45 

B 

Bach  inger 203 

Barrois 201 

Barium-mercury  solution 75 

Bastite 22,  150,  192,  200 


Becke 92,  164,197-211,213,  214 

Becker 202,  204,  207,  208 

Behrens 51,59,200,208,210 

Belonite 87 

Bertrand 7 

Biotite 36,  89,  156 

Bitumen no 

Blaas..   205,  2xi,  213 

Bodewig    203 

Boh  m 198 

Boricky 51,  55,  207-210 

Bourgeois 51 

Brbgger 208 

Bronzite 22,  146,  200 

Buckrng 200 

Biitschly 199 

Bytownite 186,  200 


Cadmium-boro-tungstate  solution 73 

Caesium  alum 63 

Calcite..   38,  132,  200 

Calcite  plate 12 

Calderon 7 

—  Double-plate 12 

Calderon  y  A  ratio. 201 

Carinthine 172 

Cancrinite 136,  200 

Catkrein 76.  210-212 

Centring  adjustment  on  microscope 13 

Chabazite 194 

Chalcedony 192.  2°° 

Chemical  investigation.  Methods  of 50 

Chiastolite 152*  201 

Chlorite 162 

Chloritoid 162,  201 

Chlorophaeite 192,  213 

Chromite no,  118,  201 

Ch  rustschoff 99,  205,  aio,  213 

Chnochlore 162 


230 


INDEX. 


PAGE 

Cohen i,  66,  81,  85,  197,  205,  208,  209 

Condenser 10 

Cordierite 40,  47,  154,  201 

Corrosion  of  the  rock-forming  minerals. .     88 

Corundum 138 

Couseranite 126,  201 

Cross 198,  201,  204,  205,  213 

Crystal  formation,  Disturbances  in  the..     87 

Crystallites 86 

Crystallization,  Determination  of  the  sys- 
tems of 106 

Crystalloids 86 

Cyanite 178,  203 


Dathe 198-202,  207,  208,  211-213 

Decomposition  of  the  rock-constituents. .  101 

Delessite 192,  213 

Des  Cloizeaux 207 

Determination  of  the  system  of  crystal- 
lization of  the  rock-forming  minerals. . .  106 

Diaclasite 150,  210 

Diallage 170,  201 

Dichroite 154 

Diller 212 

Dio'pside 170,  202 

Dipyr 126,  201 

Dispersion  of  the  optic  axes 36 

Disthene  29,  178,  202 

Doelter 79,  204,  210 

Dolomite  132,  202 

Double-refracting    minerals    in    parallel 

polarized  light 17 

Double-refraction,  Determination  of  the 

character  of 31,  34 

v.  Drasche 198-200,  202,  211 


E 

Eisen-kies 108 

Elseolite 134,  203 

Enstatite 21,  144,  203 

Epidote 24,42,  176,  203 

Extinction,  Oblique 25 

,  Parallel 19 


Feldspar,  Shell-formed  structure  of 91 

,  Decomposition  of 102 

Fitter 197 

Fletcher 213 

Fluid  inclosures 94.  95 


PAGE 

Fluor-spar  (Fluorite) 120,  203 

Form  of  occurrence  of  the  rock-compo- 
nents      81 

v.  Foullon 208 

Fouque i,  66,  76,  79,  81,  197,  205,  206,  213 

Fourth-undulation  mica 13 


Gas-pores  ............................  ... 

Gastaldite  ............................... 

Gisevius  ................................. 

Glasmasse    ............................ 

Glaucophane  ..........................  174, 

Globulite  ............................... 

Goldschmidt  ...........................  66, 

Garnet  .................................. 

Graphite  .............................  no, 

v.  Groddeck  ............................. 

Groth  .............................  16,  45, 

Green  earth  ............................ 

Gum  bee  .........  ........................ 

Gylling  .................................. 

Gypsum  ..............................  178, 


67 
n2 
203 

86 
201 
u6 
204 
207 
212 
ig2 
205 
211 
204 


Hagge 199,  202,  204,  205,  208,  209 

Hague 205 

Hammer schmidt 204 

Harada's  apparatus 70 

Hare. . .  ..   an 


Hausmann 203 

Hauyn 54,  u4,  204 

Heating  apparatus  15 

Helminth  162,  210 

Hematite no,  138,  204 

Hercynite 118,  204 

Hexagonal  minerals 106,  128 

,  Behavior  of,  in  polarized  Iight.i8,  30 

Hopfner 210 

Hollrung 210 

Hornblende,  Cleavage  of  84 

,  Ordinary  and  basaltic 172 

,  Opacitic  border  of 89 

— ,  Optical  orientation  of  ...   24,    28 

Humboldilith 126 

Hussak 201,  206,  209,  2ii 

Hydrofluosilicic  acid 55 

Hypersthene 148,  204 

,  Inclosures  in 100 

,  Optical  orientation  of 22 

,  Pleochroism  of 48 


INDEX. 


231 


PAGE 
I 

Iddings 205 

Ilinenite  no 

Index  of  refraction.  Determination  of.  14,    44 
Inclosures  of  the  rock-forming  minerals..     93 

of  gases 94 

of  fluids 95 

—  of  vitreous  particles 97 

—  of  foreign  minerals 99 

Inostranzeff 200,  202 

Interference-figures 32 

Interpenetration  of  the  rock-constituents    93 

Investigation,  Optical  methods  of  16 

• ,  Chemical  methods  of 50 

Isotrope 106 

J 

Jeremejeff. 198 

v.  John 198,  203,  205,  210 

K 

ii.  Kalkowsky 202-204,  206,  211,  212 

Kaemmente 162 

Kispatlc - 208 

C.  Klein 7 

D.Klein 66,  73 

Kleiiis  solution 73 

Klocktnann 210 

Kloos 202,  204,  207,  213 

Knop 54,  206 

V.  Kobell 202 

Koch ..197,200,203,212 

Koenig 199 

Koller 207 

Kosmann  204,  213 

Kren  ner 205 

Kreutz 199,  205,  208,  210 

Kiich 203 

L 

Labradorite 186,  205 

Lagorio 199,  200,  213 

v.  Lasaulx..  .7,  198,  201-204,  207»  209i  211-213 

Lnspeyres 7,  45,  49,  203,  205,  208 

Lehmann 210 

Lemberg  200,  201,  211 

Leucite 120,122,205 

Liebenerite 136,  206 

Liebisch 7 

Longulite 86 

Lessen 198,207 

L  uedecke 203 


M 

Magnesite 132,  206 

Magnetite 108,  206 

Magnet-kies  no 

Maly   212 

Mann 197,  212 

Margarite 86 

Measurement  of  angles 83 

Mechanical  separation  of  the  rock-forming 

minerals 66 

by  means  of  solutions  of  high 

specific  gravity 67 

by  the  solution  of  the  iodides  of 

barium  and  mercury 75 

by  the  solution  of  the  iodides  of 

potassium  and  mercury 67 

by    the    solution    of    cadmium 

boro-tungstate 73 

by  means  of  the  electro-magnet.    79 

by  means  of  acids 76 

,  Apparatus  for 70 

Meionite 124,  206 

Melanite 116,  206 

Mellilith 126,  206 

Meroxene 156,  206 

Meyer  200,  202,  210-213 

Michel Le"vy ...i,  28,  44,  51,  66,  76,  81,  197,  207, 

211,  214 

Microchemical  Methods 51 

of  Boricky 55 

of  Behrens 59 

Microchemical  reactions  with  Aluminium    62 

Barium 63 

Boron 65 

Calcium 57,  60 

Chlorine 64 

:  Fluorine 64 

Iron 57,  63 

Lithium 57.  63 

Magnesium  57,  62 

— Manganese 58,  63 

Phosphorus 64 

Potassium 56,  60 

Silicon 65 

Sodium 56,61 

—  Strontium %  58,  63 

Sulphur 64 

Water 66 

Microcline 180,  207 

!   Microlites 85,86,207 

|   Micrometer 14 


232 


INDEX. 


PAGE 

Micropegmatite 93 

Microperthite 180 

Microscope 7 

Monoclinic  minerals 107 

,  Behavior  of,  in  pol.  light.  .23,  36,  40 

Morphological    properties    of    the    rock- 
forming  minerals 81 

Miigge 197,207 

Mutter 198,201 

Muscovite 36,160,207 

N 

Natrolite  194 

Nepheline 53,  134,  207 

ffessig 214 

PJiedzwiedzky 205 

Nigrine 122 

Non-pellucid  minerals 108 

Nosean 114,  207 


Ocular  micrometer 14 

Oebbtke 66,76 

Oligoclase 184,  207 

: Albite 182,208 

Olivine 45,  89,  140,  208 

; ,  Decomposition  of 101 

Omphacite 170,  202 

Opacitic  border 89 

Opal  112,  208 

Optically-uniaxial  minerals 18,  30,  46,  106 

Optically-biaxial  minerals 20,  32,  47,  107 

Orthoclase  25,42,164,  208 

Oschatz 200 

Ottrelite 164,  209 

P 

Penck 208,  209 

Penninite 162 

Perowskite  120,  209 

Peters 212 

Pfaff 209 

Phlogopite 158,  209 

Pichler 211,213 

Picotite 118,  209 

Finite 154,  209 

Plagioclase 209 

,  Shell-formed  structure  of 91 

,  Twins  of 43,  44 

Pleochroism  45 

Pleonaste  no,  118,  210 

Pohlig 198,  201 

Polarization-microscope 7,  8 


PACK 

Polarizer 7 

Potassium  fluo-borate 61 

Mercury  solution 67 

Platinum  chloride 61 

Preparation  of  microscopical  sections 3 

Prism,  Nicol's 7- 

Protobastite  150,  210 

Pseudo-crystals ...  89 

Pyrite 108 

Pyrope 116,  210 

Pyrrhotine no 

0 

Quartz 88,  128,  210 

wedge 13 

plate,  Biot-Klein's n 

R 

v.  Rath 198,  202,  205-207,  209,  2ii 

Regular  minerals  106,  u4 

,  Behavior  of,  in  pol.  light 17,  30 

Renard 165,  198,  200,  202,  204,  209 

Reusch 200,  203,  208 

Rhombic  minerals * 107,  140 

— ,  Behavior  of,  in  pol.  light 21,  35 

Riess 198,  202,  204,  214 

Ripidolite 162,  210 

Rohrbach 67,  75 

Rose 201,  204,  208,  209,  213 

Rosenbusch.   i,  7,  16,  45,  51,  76,  81,  92,  197-200, 
205,  207-210,  213 

Roth. ., loi.  an 

Rubellan 158,  210 

Rutile 38,  122,  210 


Sagenite  122 

Salite 170,  202 

Sandberger 205,211,  213 

Sanidine 166,  208 

Snuer 204,  205,  210 

Scapolite 124,  211 

Scolecite 194 

Scheerer  203 

Schonn 51 

Schorl 138 

Schrauf 198,  200,  202,  205,  208,  210,  211 

Schultze 13.208 

Schulze 2ir 

Schumacher 211 

Schuster 198,  200,  205,  207,  209,  213 

Sericite 160 

Serpentine - 190,  211 


INDEX. 


233 


PAGE 

Shell-formed  structure  of  crystals  90 

Siderite 132 

Silico-fluorides 56,  57,  58 

Sillimanite. 142,  211 

Single-refracting  minerals 17 

Sip'ocz 201,  209 

Sismondme 162,  201 

Sjogren 199 

Smaragdite 172,  174 

Sodahte 114,  211 

Somuterlad 204 

Sorby   ...44,210 

Specific  gravity,  Determination  of 68 

Spinel 118 

Stage,  Healing 15 

—  of  the  polarization-microscope 7 

—  scale    14 

Staurolite 39,143,212 

Stauroscopic  apparatus 7,  13 

Stelzner 200,  203,  205,  206,  209 

Stilbite 194 

Streng 51,  200,  202,  203,  205,  208,  210,  212 

Structure  of  the  rock-forming  minerals...    87 
Szabo 51,  198,  201 


Talc 160,212 

Teller 198,  203,  205,  210 

Tetragonal  minerals 106,  122 

,  Behavior  of,  in  pol.  light 18,  30 

Thoulet 6,  44,  66,  81,  201 

^Titanite 25,  42,  176,  212 

Titaneisen  102,110,205 

Titan  magneteisen 108,  212 


PACK 

Tornebohm >97»  200,  203,  211,  213,  214 

Tourmaline 46,  138,  213 

Tremolite 174,213 

Trichite 87 

Triclinic  minerals 107,  178 

— ,  Behavior  of,  in  pol.  light 28,  37 

Tridymite 130,  212 

Trippke 203 

Tschermak  45,  197-204,  206-213 

Twins,  Behavior  of,  in  pol.  light 37 

U 
Uralite 174,213 

V 

Valee-Poussin 209 

Velain 206,  208 

Viridite  . .  192.  2I3 

Vitreous  inclosures 97 

Vogelsang 15,  85,  204,  205,  213 

Vrba 199,  2ii 

W 

Websky 211 

Wedding 199 

Weigand 211 

Weiss 208-21 1 

v.  Werveke 66,  199,  202,  203,  210-212 

Wichman 6,  198,  201,  206,  209,  213 

Williams 203.  207 

Wollastonite ....25,  172,  213 

Z 

Zeolites 194,  213 

Zircon 124,213 

Zirkel i,  51,  76,  197-214 

Zoisite 154,  214 


MW14 


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